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|
<!DOCTYPE html>
<html lang=en>
<head>
<title></title>
<meta charset="utf-8"/>
<link rel="shortcut icon" href="favicon.ico"/>
<link rel='stylesheet' href='../style.css'/>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<header><h1>Postfix and Dovecot Mail Server</h1></header>
<main>
<p>Postfix and dovecot will be the two primary pieces of our mail sever.
Postfix is the mail transport agent that handles the sending and
receiving of mail and dovecot is the IMAP server that will allow us to
access our mail from a mail client such as mutt. The server will also
have several other supporting components, a complete list of which is:</p>
<ul>
<li><em>SpamAssassin</em> for spam filtering</li>
<li><em>OpenDKIM</em> for DKIM verification and signing</li>
<li><em>Postgrey</em> for greylisting</li>
<li><em>Policyd-SPF</em> for SPF verification</li>
<li><em>OpenDMARC</em> for DMARC verification</li>
</ul>
<p>You can use <a href=https://git.chudnick.com/mail-tools/tree/server/deploy-server>
this script</a> I have written to automate this process, but I would
recommend that you run through the tutorial first to understand
what is being done.</p>
<p>Please note that this tutorial is loosely intended for small personal mail
servers. Using PAM for authentication, as is done here, is not a scalable solution
for working with a large number of users. I do plan on covering Dovecot LDAP
authentication at some point which would be a better solution in an enterprise
setting.</p>
<h2>Install Packages</h2>
<p>Let's start by installing the required packages. Note that if you already
have Apache installed on the server, replace <em>python3-certbot-nginx</em>
with <em>python3-certbot-apache</em>.</p>
<pre><code>apt install postfix dovecot-imapd dovecot-sieve opendkim opendkim-tools spamassassin gnupg postgrey postfix-policyd-spf-python opendmarc dbconfig-no-thanks certbot python3-certbot-nginx</code></pre>
<p>During the installation of Postfix you will get a Debconf prompt in which
you need to select "Internet Site" and then provide your domain name,
<strong>example.com</strong>.</p>
<h2>Get a certificate</h2>
<p>Now we'll use Certbot to get a certificate for our server. If you are
using Apache replace <em>nginx</em> with <em>apache2</em>.</p>
<pre><code>systemctl stop nginx
certbot certonly --standalone -d <strong>mail.example.com</strong>
systemctl start nginx</code></pre>
<h2>Postfix Main Configuration</h2>
<p>In this section we will be doing the bulk of the postfix configuration.
The postconf command used throughout appends (or changes)
the specified configuration item in /etc/postfix/main.cf</p>
<h3>Network Configuration</h3>
<p>Let's start by configuring some network and domain information.</p>
<pre><code>postconf -e "myorigin = <strong>example.com</strong>"
postconf -e "mydestination = \$myhostname, \$mydomain, localhost"
postconf -e "mynetworks = 127.0.0.0/8 [::1]/128"
postconf -e "myhostname = <strong>mail.example.com</strong>"
</code></pre>
<p>Next, point postfix to the cerbot key and certificate, as well as the distro's
CA certificates.</p>
<pre><code>postconf -e "smtpd_tls_key_file=/etc/letsencrypt/live/<strong>mail.example.com</strong>/privkey.pem"
postconf -e "smtpd_tls_cert_file=/etc/letsencrypt/live/<strong>mail.example.com</strong>/fullchain.pem"
postconf -e "smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt"
</code></pre>
<p>Harden the TLS configuration by forcing strong
protocols and ciphers, and requiring that authentication occur only over an
encrypted session.</p>
<pre><code><em># Require authentication over TLS and optionally use it for sending and receiving mail</em>
postconf -e "smtpd_tls_auth_only = yes"
postconf -e "smtpd_tls_security_level = may"
postconf -e "smtp_tls_security_level = may"
<em># Force the use of TLSv1.2 or TLSv1.3</em>
postconf -e 'smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1'
postconf -e 'smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1'
postconf -e 'smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1'
postconf -e 'smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1'
<em># Prefer server ciphers</em>
postconf -e "tls_preempt_cipherlist = yes"
<em># Force strong ciphers</em>
postconf -e "smtpd_tls_ciphers = high"
postconf -e "smtpd_tls_mandatory_ciphers = high"
postconf -e "smtp_tls_ciphers = high"
postconf -e "smtp_tls_mandatory_ciphers = high"
postconf -e "smtpd_tls_exclude_ciphers = aNULL, eNULL, EXP, LOW, MEDIUM, PSK, SRP, SHA1, kRSA, CAMELLIA, ARIA, DSS, RSA+AES, ADH, AECDH"
postconf -e "smtp_tls_exclude_ciphers = aNULL, eNULL, EXP, LOW, MEDIUM, PSK, SRP, SHA1, kRSA, CAMELLIA, ARIA, DSS, RSA+AES, ADH, AECDH"
</code></pre>
<h3>Local Recipients and Aliases</h3>
<p>Here we configure the bulk of the postfix built-in security settings which are
structured as a series of access restrictions. Do not edit these settings without
first reading the Postfix documentation as an incorrect change could inadvertently
make your server an open relay.</p>
<pre><code>postconf -e "smtpd_helo_required = yes"
postconf -e "smtpd_sender_login_maps = proxy:hash:/etc/postfix/login_maps"
postconf -e "smtpd_helo_restrictions = reject_unknown_helo_hostname, reject_non_fqdn_helo_hostname"
postconf -e "smtpd_sender_restrictions = reject_sender_login_mismatch, reject_non_fqdn_sender, reject_unknown_sender_domain"
postconf -e "smtpd_recipient_restrictions = reject_non_fqdn_recipient, reject_unknown_recipient_domain, permit_sasl_authenticated, reject_unauth_destination, check_policy_service unix:private/postgrey, check_policy_service unix:private/policyd-spf, reject_rbl_client zen.spamhaus.org"
postconf -e "smtpd_relay_restrictions = permit_sasl_authenticated, reject_unauth_destination"
postconf -e "smtpd_data_restrictions = reject_unauth_pipelining"
<em># Disable VRFY command to prevent harvesting of user accounts on system</em>
postconf -e "disable_vrfy_command = yes"
<em># Change smptd banner (hide distribution)</em>
postconf -e "smtpd_banner = \$myhostname ESMTP \$mail_name"</code></pre>
<p>Now, configure the local mail recipients and some aliases. We'll create
an account called <strong>mailadmin</strong> to receive mail addressed to
several other accounts. This is to keep <em>administrative</em> mail separate, but
you can certainly alias these to your main account later if you would prefer to see
it there.</p>
<pre><code><em># Set a custom local_recipient_maps here in order to avoid accepting mail for all local accounts</em>
postconf -e "local_recipient_maps = proxy:hash:/etc/postfix/local_maps \$alias_maps"
<em># You will need to manually set a password later to login as mailadmin</em>
adduser --disabled-login --shell /usr/sbin/nologin --gecos "" mailadmin
echo "# postfix aliases
postmaster: mailadmin
root: mailadmin
dmarc: mailadmin
" > /etc/aliases
<em># Update address databases</em>
echo "mailadmin@<em>mail.example.com</em> mailadmin" > /etc/postfix/login_maps
echo "mailadmin mailadmin" > /etc/postfix/local_maps
newaliases
postmap /etc/postfix/login_maps
postmap /etc/postfix/local_maps
</code></pre>
<h3>Mail Delivery</h3>
<p>These commands configure our mail delivery preferences. Mail will be
delivered inside a user's home folder with a maildir-style mailbox using
dovecot.</p>
<pre><code><em># Maildir delivery to $HOME/Mail/Inbox/</em>
postconf -e "home_mailbox = Mail/Inbox/"
<em># Deliver mail with Dovecot</em>
postconf -e "mailbox_command = /usr/lib/dovecot/deliver"</code></pre>
<h3>Header and Body Checks</h3>
<p>Header and body checks allow for some simple content filtering within Postfix.
This is done by scanning a message line by line for a configured regex string,
nothing more. For example, the first header check listed will reject a message
with an attachment of <em>ransomware.exe</em> but will not block it if sent with
no extension. This is mostly a protection against uneducated users and poorly
written mail clients. Other checks block vulnerabilities and improve privacy.</p>
<p>Create a new file <strong>/etc/postfix/header_checks</strong>, then open it in a
text editor and add the following</p>
<pre><code><em># Block files with common executable extensions</em>
/name=[^>]*\.(exe|pif|com|dll|vbs|bat|sh|bash|so|zip|tar|gz|cpio)/ REJECT
<em># Block message/partial vulnerability</em>
/message\/partial/ REJECT
<em># Remove Received string that is created when spamassassin reinjects message into postfix</em>
<em># This is to prevent leaking the userid of the spamassassin user</em>
/^Received:.*userid.*/ IGNORE
<em># Remove User-Agent strings from headers</em>
/^User-Agent: .*/ IGNORE</code></pre>
<p>Create another new file <strong>/etc/postfix/body_checks</strong>, and add this</p>
<pre><code><em># Block messages with iframes</em>
/<iframe/ REJECT" > /etc/postfix/body_checks</code></pre>
<p>And then run these commands to point postfix to the check files.</p>
<pre><code>postconf -e "header_checks = regexp:/etc/postfix/header_checks"
postconf -e "body_checks = regexp:/etc/postfix/body_checks"</code></pre>
<h2>Postfix Master Configuration</h2>
<h3>SMTP client</h3>
<p>This simple command configures the SMTP client process that is responsible
for sending your mail to other mail servers.</p>
<pre><code>postconf -M "smtp/unix=smtp unix - - y - - smtp"</code></pre>
<h3>Postscreen and SMTP Recipient</h3>
<p>Postscreen is a kind of firewall that sits in front of the Postfix SMTPD
process and receives all incoming traffic. Postscreen will drop connections
from IPs on a DNS blacklst, or from clients that violate the SMTP protocol by
speaking out of turn or sending non-SMTP commands. This adds up to less spam
connections and therefore a much lighter workload for your server.</p>
<pre><code>postconf -M "smtp/inet=smtp inet n - y - 1 postscreen"
postconf -M "smtpd/pass=smtpd pass - - y - - smtpd"
postconf -P "smtpd/pass/content_filter=spamassassin"
postconf -M "tlsproxy/unix=tlsproxy unix - - y - 0 tlsproxy"
postconf -M "dnsblog/unix=dnsblog unix - - y - 0 dnsblog"
postconf -e "postscreen_dnsbl_sites = zen.spamhaus.org"
postconf -e "postscreen_dnsbl_action = enforce"
postconf -e "postscreen_greet_action = enforce"
</code></pre>
<h3>Submission over TLS (submissions)</h3>
<p>Submission over TLS (aka submissions) is the process you will use to submit
mail to your server from a mail client. These commands configure submissions to
use a fully-encrypted session, as opposed to STARTTLS, and to only allow access
to authenticated clients.</p>
<pre><code>postconf -M "submissions/inet=submissions inet n - y - - smtpd"
postconf -P "submissions/inet/smtpd_tls_wrappermode=yes"
postconf -P "submissions/inet/smtpd_tls_security_level=encrypt"
postconf -P "submissions/inet/smtpd_tls_auth_only=yes"
postconf -P "submissions/inet/smtpd_sasl_auth_enable=yes"
postconf -P "submissions/inet/smtpd_client_restrictions=permit_sasl_authenticated,reject"
postconf -P "submissions/inet/smtpd_helo_restrictions="
postconf -P "submissions/inet/smtpd_sender_restrictions=reject_sender_login_mismatch"
postconf -P "submissions/inet/smtpd_recipient_restrictions=reject_non_fqdn_recipient,reject_unknown_recipient_domain,permit_sasl_authenticated,reject"
postconf -P "submissions/inet/syslog_name=postfix/submissions"
postconf -P 'submissions/inet/smtpd_tls_protocols=!SSLv2,!SSLv3,!TLSv1,!TLSv1.1'
postconf -P 'submissions/inet/smtpd_tls_mandatory_protocols=!SSLv2,!SSLv3,!TLSv1,!TLSv1.1'</code></pre>
<h3>OPTIONAL - submission with mandatory STARTTLS</h3>
<p>Having configured submission over TLS on port 465 this step is optional.
STARTTLS is considered by some to be less secure than full-session TLS and
may be vulnerable to exploitation.</p>
<pre><code>postconf -M "submission/inet=submission inet n - y - - smtpd"
postconf -P "submission/inet/smtpd_tls_security_level=encrypt"
postconf -P 'submission/inet/smtpd_tls_protocols=!SSLv2,!SSLv3,!TLSv1,!TLSv1.1'
postconf -P 'submission/inet/smtpd_tls_mandatory_protocols=!SSLv2,!SSLv3,!TLSv1,!TLSv1.1'
postconf -P "submission/inet/smtpd_sasl_auth_enable=yes"
postconf -P "submission/inet/smtpd_tls_auth_only=yes"
postconf -P "submission/inet/syslog_name=postfix/submission"
postconf -P "submission/inet/smtpd_helo_restrictions="
postconf -P "submission/inet/smtpd_client_restrictions=permit_sasl_authenticated,reject"
postconf -P "submission/inet/smtpd_helo_restrictions="
postconf -P "submission/inet/smtpd_sender_restrictions=reject_sender_login_mismatch"
postconf -P "submission/inet/smtpd_recipient_restrictions=reject_non_fqdn_recipient,reject_unknown_recipient_domain,permit_sasl_authenticated,reject"
</code></pre>
<h3>SpamAssassin Configuration</h3>
<p>Finally, this command tells Postfix how to interact with SpamAssassin.</p>
<pre><code>postconf -M "spamassassin/unix=spamassassin unix - n n - - pipe user=debian-spamd argv=/usr/bin/spamc --socket=/var/run/spamd.sock -e /usr/sbin/sendmail -oi -f \${sender} \${recipient}" </code></pre>
<h2>Dovecot Configuration</h2>
<p>Dovecot configuration is usually split up into many different files under
<strong>/etc/dovecot/conf.d/</strong> but here will be doing all of the
configuration in the primary config file
<strong>/etc/dovecot/dovecot.conf</strong>. Open that file with your editor
of choice, clear all of its contents, and then replace it with the following.</p>
<pre><code><em># /etc/dovecot/conf.d/10-auth.conf</em>
disable_plaintext_auth = yes
auth_username_format = %n
auth_mechanisms = plain
userdb {
driver = passwd
}
passdb {
driver = pam
}
<em># /etc/dovecot/conf.d/10-mail.conf</em>
mail_location = maildir:~/Mail:INBOX=~/Mail/Inbox:LAYOUT=fs
namespace inbox {
type = private
prefix =
separator = /
inbox = yes
subscriptions = yes
list = yes
}
<em># /etc/dovecot/conf.d/10-master.conf</em>
service imap-login {
# Run login processes in high-security mode (see: LoginProcess.txt in dovecot docs)
service_count = 1
# Disable unencrypted IMAP by setting port for plain IMAP to 0
inet_listener imap {
port = 0
}
inet_listener imaps {
port = 993
ssl = yes
}
}
<em># Allow postfix to use dovecot SASL</em>
service auth {
unix_listener /var/spool/postfix/private/auth {
mode = 0660
user = postfix
group = postfix
}
}
<em># /etc/dovecot/conf.d/10-ssl.conf</em>
ssl = required
ssl_key = </etc/letsencrypt/live/<strong>mail.example.com</strong>/privkey.pem
ssl_cert = </etc/letsencrypt/live/<strong>mail.example.com</strong>/fullchain.pem
ssl_client_ca_dir = /etc/ssl/certs
ssl_dh = </usr/share/dovecot/dh.pem
<em># Mozilla intermediate compatibility</em>
ssl_min_protocol = TLSv1.2
ssl_cipher_list = ECDHE+ECDSA+AESGCM:ECDHE+aRSA+AESGCM:ECDHE+ECDSA+CHACHA20:ECDHE+aRSA+CHACHA20:DHE+aRSA+AESGCM:!aNULL:!eNULL
ssl_prefer_server_ciphers = yes
ssl_client_require_valid_cert = yes
<em># /etc/dovecot/conf.d/15-lda.conf</em>
protocol lda {
mail_plugins = \$mail_plugins sieve
}
<em># /etc/dovecot/conf.d/15-mailboxes.conf</em>
namespace inbox {
mailbox Sent {
special_use = \Sent
auto = subscribe
}
mailbox Trash {
special_use = \Trash
auto = create
autoexpunge = 30d
}
mailbox Drafts {
special_use = \Drafts
auto = subscribe
}
mailbox Spam {
special_use = \Junk
auto = create
autoexpunge = 30d
}
mailbox Archive {
special_use = \Archive
auto = create
}
}
<em># /etc/dovecot/conf.d/20-imap.conf</em>
imap_capability = +SPECIAL-USE
<em># /etc/dovecot/conf.d/90-sieve.conf</em>
plugin {
sieve = ~/.dovecot.sieve
sieve_default = /var/lib/dovecot/sieve/default.sieve
sieve_global = /var/lib/dovecot/sieve/
}</code></pre>
<p>Then create the default sieve filtering script at
<strong>/var/lib/dovecot/sieve/default.sieve</strong></p>
<pre><code>require ["fileinto", "mailbox"];
/*
* Discard mail that has a spam score greater than or equal to 10
*/
if header :contains "X-Spam-Level" "**********" {
discard;
stop;
}
/*
* Discard messages marked as infected by a virus scanner
*/
if header :contains "X-Virus-Scan" "infected" {
discard;
stop;
}
/*
* If message is marked as spam (and falls below discard threshold) put into spam mailbox
*/
if header :contains "X-Spam-Flag" "YES" {
fileinto "Spam";
}</code></pre>
<p>And compile the script</p>
<pre><code>sievec /var/lib/dovecot/sieve/default.sieve</code></pre>
<p>Finally, configure PAM authentication for dovecot at
<strong>/etc/pam.d/dovecot</strong>. Append these changes leaving any include
statements intact.</p>
<pre><code>auth required pam_unix.so
account required pam_unix.so</code></pre>
<h2>OpenDKIM</h2>
<p>DKIM is a mail-verification method that cryptographically signs mail
to allow receivers to verify the authenticity of the sender. Our mail server
will use DKIM to validate signatures on incoming mail and sign outgoing mail. DKIM
requires a public key to be published via DNS, which will be done near the end of
the guide.</p>
<p>Start by generating the DKIM key</p>
<pre><code>opendkim-genkey -D /etc/dkimkeys -d <em>example.com</em> -s mail
chown opendkim: /etc/dkimkeys/*
chmod 600 /etc/dkimkeys/*
mv /etc/dkimkeys/mail.private /etc/dkimkeys/mail.pem</code></pre>
<p>Here we make a directory for the opendkim socket inside the postfix chroot and
make it accessible to the postfix user.</p>
<pre><code>mkdir /var/spool/postfix/opendkim
chmod 770 /var/spool/postfix/opendkim
chown opendkim:opendkim /var/spool/postfix/opendkim
usermod -aG opendkim postfix</code></pre>
<p>Edit the configuration file at <strong>/etc/opendkim.conf</strong>
to be as follows:</p>
<pre><code>On-BadSignature reject
On-Security reject
Syslog yes
SyslogSuccess yes
LogResults yes
Canonicalization simple
Mode sv
OversignHeaders From
Domain <strong>example.com</strong>
Selector mail
KeyFile /etc/dkimkeys/mail.pem
UserID opendkim
UMask 007
Socket local:/var/spool/postfix/opendkim/opendkim.sock
PidFile /run/opendkim/opendkim.pid
TemporaryDirectory /run/opendkim
InternalHosts 127.0.0.1
TrustAnchorFile /usr/share/dns/root.key
RequireSafeKeys True
AlwaysAddARHeader True
</code></pre>
<h2>OpenDMARC</h2>
<p>DMARC is another mail-verification technology that provides verification of the
address seen by end-users and either or both of SPF and DKIM.
<p>Like with OpenDKIM, we need to make a directory inside the postfix chroot
for the socket and assign proper permissions.</p>
<pre><code>mkdir /var/spool/postfix/opendmarc
chmod 770 /var/spool/postfix/opendmarc
chown opendmarc:opendmarc /var/spool/postfix/opendmarc
usermod -aG opendmarc postfix
</code></pre>
<p>Now we write the configuration file at <strong>/etc/opendmarc.conf</strong></p>
<pre><code>PidFile /run/opendmarc/opendmarc.pid
PublicSuffixList /usr/share/publicsuffix/public_suffix_list.dat
RejectFailures True
Socket local:/var/spool/postfix/opendmarc/opendmarc.sock
Syslog True
SyslogFacility mail
UMask 002
UserID opendmarc
HistoryFile /var/run/opendmarc/opendmarc.hist
SPFIgnoreResults True
SPFSelfValidate True
</code></pre>
<p>Then create the history file and set permissions.</p>
<pre><code>touch /var/run/opendmarc/opendmarc.hist
chown opendmarc:opendmarc /var/run/opendmarc/opendmarc.hist
chmod 664 /var/run/opendmarc/opendmarc.hist
</code></pre>
<p>Now that both OpenDKIM and OpenDMARC are configured we can define them as milters
in postfix. This will tell postfix to route mail through one or both of these milters
depending on whether it is incoming or outgoing.</p>
<pre><code>postconf -P "smtpd/pass/smtpd_milters=unix:opendkim/opendkim.sock,unix:opendmarc/opendmarc.sock"
postconf -P "submissions/inet/smtpd_milters=unix:opendkim/opendkim.sock"
<em># If you enabled submission on port 587 run this too</em>
postconf -P "submission/inet/smtpd_milters=unix:opendkim/opendkim.sock"
</code></pre>
<h2>Postgrey</h2>
<p>Postgrey implements a spam-filter technique known as greylisting, which
always rejects mail on the first try and for a period of time afterwards known
as the greylist period. The idea behind this being that legitimate senders will
send the mail again later, while spammers, in a rush to send as many messages as
possible before being blacklisted, will not.</p>
<p>Postgrey ships with an extensive whitelist domains that are known
to cause issues (mainly large providers that constantly send from different
addresses). This whitelist file is located at
<strong>/etc/postgrey/whitelist_clients</strong> and can be appended to include
any domain you do not wish to be subject to greylisting.</p>
<p>The configuration needed here is minimal, just open
<strong>/etc/default/postgrey</strong> and make these changes</p>
<pre><code>POSTGREY_OPTS="--unix=/var/spool/postfix/private/postgrey --privacy"
POSTGREY_TEXT="Greylisted - see https://www.greylisting.org"</code></pre>
<p>And then enable the service</p>
<pre><code>systemctl enable --now postgrey</code></pre>
<h2>Policyd-SPF</h2>
<p>SPF is yet another mail-verification technology that uses DNS records to
delegate specific servers as being authorized to send mail for the domain
(and implicitly all other servers as unauthorized). Policyd-SPF will perform
SPF checking of received mail and reject mail that fails SPF verfication.</p>
<p>First, tell postfix how to access Policyd-SPF</p>
<pre><code>postconf -e "policyd-spf_time_limit = 3600"
postconf -M "policyd-spf/unix=policyd-spf unix - n n - 0 spawn user=policyd-spf argv=/usr/bin/policyd-spf"</code></pre>
<p>And then edit the configuration file at
<strong>/etc/postfix-policyd-spf-python/policyd-spf.conf</strong></p>
<pre><code>debugLevel = 1
TestOnly = 1
HELO_reject = Fail
Mail_From_reject = Fail
Header_Type = AR
<em># These settings increase false-positive risk</em>
<em># Comment them if you want to reduce that risk</em>
PermError_reject = True
TempError_Defer = True</code></pre>
<h2>SpamAssassin</h2>
<p>SpamAssassin is a spam-filter that will scan all received mail and assign
a spam score based on configured rules. SpamAssassin is much heavier and more
resource-intensive than any of the previous spam-filtering/verification programs
we have configured. The postfix spam-filtering philosophy emphasizes the use
of lightweight checks before passing to an external content filter such as
SpamAssassin. Ideally, non-legitimate mail will have already been caught by one
of the previous methods, and SpamAssassin will only have to operate on a much
smaller subset of the mail that is sent to our server.</p>
<p>We have actually already told postfix to use SpamAssassin as a content filter
so in this section we just need to edit the configuration file
<strong>/etc/spamassassin/local.cf</strong>.</p>
<pre><code><em># Clearly indicate message is spam to user</em>
rewrite_header Subject *****SPAM*****
rewrite_header From *****SPAM*****
<em># Set required score to be marked as spam, 5.0 is default.</em>
<em># Lower to make policy more strict or raise to be more lenient.</em>
required_score 5.0
<em># Attach original messages as text/plain instead of message/rfc822 to spam reports</em>
report_safe 2
<em>Do not implicitly trust mail based on IP address except localhost</em>
trusted_networks 127.0.0.1/32
</code></pre>
<p>And finally make a few changes to the defaults file at
<strong>/etc/default/spamassassin</strong></p>
<pre><code>OPTIONS="--listen /var/run/spamd.sock --max-children 5"
PIDFILE=/var/run/spamd.pid
CRON=1</code></pre>
<h2>Wrapping Up</h2>
<p>At this point we have done all of the necessary configuration of the mail
server programs. We have just a few more minor tasks before your mail server
is operational.</p>
<h3>Configure Firewall</h3>
<p>We need to open the proper ports in the firewall. This example uses UFW.</p>
<pre><code>ufw allow 25 comment "smtp"
ufw allow 465 comment "submission over TLS"
<em># Run this next command only if you enabled submission on port 587</em>
ufw allow 587 comment "mail submission"
ufw allow 993 comment "IMAP over TLS"
ufw reload</code></pre>
<h3>Restart services</h3>
<p>Now let's restart the services to pick up any configuration changes.</p>
<pre><code>systemctl restart postfix
systemctl restart dovecot
systemctl restart opendkim
systemctl restart opendmarc
systemctl enable --now spamassassin
systemctl restart spamassassin
systemctl restart postgrey</code></pre>
<h3>DNS Entries</h3>
<p>Finally, we needs to set some required DNS records to enable mail flow and
verification. Begin by logging into your registrar or DNS host and editing
your DNS records.</p>
<h3>A Record</h3>
<p>If you did not set a wildcard A record earlier, you will need to set one now
for <strong>mail</strong>.
Alternatively, if you are running the mail server on the same server as your
website, you may want to instead make a CNAME record pointing mail to www.</p>
<h3>MX Record</h3>
<p>MX records tell servers attempting to send you mail where to send it. Open the
MX records section on your registrar and add a new record. An MX
record consists of a priority and a destination. Set the priority to 10 and the
destination to <strong>mail</strong>, or whatever your subdomain for this mail
server is. The host value can be left blank or may need to be set to "@"
depending on your registrar.</p>
<h3>DKIM TXT Record</h3>
<p>Now we will set the three TXT records we need. Open the TXT records tab on
your registrar.</p>
<p>We'll set the DKIM record first. The command we ran to
generate our DKIM keys also generates a DNS record for us which will be helpful
here. Print that to the screen with:</p>
<pre><code>cat /etc/dkimkeys/mail.txt</code></pre>
<p>You should get a lengthy output that looks something like the following. The
bolded portion is the value.</p>
<pre><code>mail._domainkey IN TXT ( <strong>"v=DKIM1; h=sha256; k=rsa; "
"p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAz50PSYL0Ob+OlF/0B77rwlzLe7zF6JKnxQNtMqcOCZ0Dar2FPhSUSz1FR0YmNuoShjMogdgKeojIzgRUqwK5GZ5Lz456qiXWkfAtLPc6UQ/WPoyEBGbJpRBYPGWdN4VoNcHkk/I4csvXW6MOI55ghPOwDmootPkCzNPR6gmNAXMe0duS4Lb+bIjy9QMOxGYVUaQ/b+7xar+fWw"
"bA3DjQa3jTLCydzzJpjEMfVaKqNhQ4N+ve7O2Mb3LF5k5B977mtok/6POjVG5HY8g6Pba+GzMFItR6nJO5EE2fyfv6cNbRLsZiM+WQmqvDBst5ejaeapy86F5PdJFlX/TUgXjtuwIDAQAB"</strong> ) ; ----- DKIM key mail for example.com</code></pre>
<p>You can cleanup the spacing of the value as your registrar should automatically
handle any needed splitting of the record. The parts you need to paste into your
registrar's web interface should then look like this.</p>
<pre><code><em># Name/Host</em>
mail._domainkey
<em># TXT Value</em>
"v=DKIM1; h=sha256; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAz50PSYL0Ob+OlF/0B77rwlzLe7zF6JKnxQNtMqcOCZ0Dar2FPhSUSz1FR0YmNuoShjMogdgKeojIzgRUqwK5GZ5Lz456qiXWkfAtLPc6UQ/WPoyEBGbJpRBYPGWdN4VoNcHkk/I4csvXW6MOI55ghPOwDmootPkCzNPR6gmNAXMe0duS4Lb+bIjy9QMOxGYVUaQ/b+7xar+fWwbA3DjQa3jTLCydzzJpjEMfVaKqNhQ4N+ve7O2Mb3LF5k5B977mtok/6POjVG5HY8g6Pba+GzMFItR6nJO5EE2fyfv6cNbRLsZiM+WQmqvDBst5ejaeapy86F5PdJFlX/TUgXjtuwIDAQAB"</code></pre>
<h3>DMARC TXT Record</h3>
<p>The DMARC record should be as follows:</p>
<pre><code><em># Name/Host</em>
_dmarc
<em># Value</em>
"v=DMARC1; p=reject; rua=mailto:dmarc@<strong>example.com</strong>; fo=1"
</code></pre>
<h3>SPF Record</h3>
<p>Your SPF record will look like this. Remember to replace
<strong>mail.example.com</strong> with your server name.</p>
<pre><code><em># Name/Host</em>
@
<em># Value</em>
"v=spf1 a:<strong>mail.example.com</strong> -all"
</code></pre>
<h3>PTR Record</h3>
<p>Many mail servers rely on PTR records for verification purposes so we need
to make sure our server's IP address resolves to the proper domain name. If
your mail server is residing on a VPS, you will need to add this record on your
VPS provider's interface, consult their documentation for details.</p>
<h2>Creating your own Mail User</h2>
<p>Your mail server is now up and running. Let's create an email for you to
receive mail.</p>
<pre><code>useradd --shell /usr/sbin/nologin --create-home --user-group <strong>user</strong>
echo "<strong>user@example.com user</strong>" >> /etc/postfix/login_maps
echo "<strong>user user</strong>" >> /etc/postfix/local_maps
postmap /etc/postfix/login_maps
postmap /etc/postfix/local_maps
postfix reload
</code></pre>
<p>I have a script available for adding and removing users that you can find
<a href=https://git.chudnick.com/mail-tools/tree/server/mailadm>here</a>.
<h3>Connecting From a Mail Client</h3>
<p>When connecting your account to a mail client you need to use these settings.</p>
<ul>
<li>Username: <strong>user@example.com</strong> </li>
<li>Password: the password for <strong>user@example.com</strong> </li>
<li>Server name: <strong>mail.example.com</strong> </li>
<li>IMAP Port: 993</li>
<li>IMAP Connection: SSL/TLS</li>
<li>SMTP Port: 465</li>
<li>SMTP Connection Type: SSL/TLS</li>
</ul>
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