Install Postfix + Dovecot e-mail server

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Note

1 Jul 2021: Adjust tls setting.
18 Jun 2021: Adjust tls setting.
5 Jan 2021: Increase imap-login process limit. List Dovecot full custom settings.
9 Dec 2020: Define cron job to delete filtered mails.
20 Sep 2020: Stop using mail-stack-delivery.
8 May 2020: Correct typo errors.
30 May 2019: Add anti-virus and spam mail filtering.
17 May 2019: Add copying emails to external accounts.
7 May 2019: Change "gksudo gedit" to "sudo gedit" as Ubuntu 18.04 dropped "gksudo". Add header_checks. Add auto creation of Trash folders. Add webmaster.
29 Sep 2018: Add "body_checks" for spam control.
20 Sep 2018: Increase message_size_limit to 20 times the default.
5 Apr 2018: Increase message_size_limit to 10 times the default.
25 Dec 2018: Publish on web.
12 Apr 2014: Specify maximal_queue_lifetime to notify unsuccessful delivery immediately.
2 Apr 2014: Specify fully qualified domain name.

Intro

Postfix is a mail transfer agent (MTA) responsible for sending out and receiving emails between servers.

Dovecot is a mail delivery agent (MDA) responsible for sending out and receiving emails between a server and its users.

Mail-stack-delivery was a combined package containing both Postfix and Dovecot. It is now no longer supported.

Re-direct to server ports

Set the internet router to re-direct the following connections to server ports:

SMTP = port 25 (for receiving or sending emails)
secure SMTP = port 465 (for receiving or sending emails securely)
IMAP = port 143 (for retrieving emails)
secure IMAP = port 993 (for retrieving emails securely)
POP3 = port 110 (for retrieving emails)

Install both Postfix and Dovecot

Ubuntu 20.04 does not support the combined package "mail-stack-delivery" anymore.

Install Dovecot and Postfix individually:

$ sudo apt install dovecot-imapd dovecot-pop3d
$ sudo apt install postfix

However, the previous config file "/etc/dovecot/conf.d/99-mail-stack-delivery.conf" is still retained for use, because "99" represents the last and overriding config file. This eliminates the need to change the individual config files.

(revised to install individually, 20 Sep 2020)

Reconfigure Postfix

Reconfig:

$ sudo dpkg-reconfigure postfix

Use Tab key to change selection.

Select "Internet Site".

Enter the following information:

System mail name: <fully qualified domain name, such as "kctang.com.hk">
Root and postmaster mail recipient: <such as "kctang">
Other destinations to accept mail: <fully qualified domain name, such as "kctang.com.hk">, <server name such as "server1">, localhost.localdomain, localhost
Force synchronous updates on mail queue: No
Local networks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
Use procmail for local delivery: No
Mailbox size limit (bytes): 0
Local address extension character: +
Internet protocols to use: all

Activate the changes:

$ sudo systemctl reload postfix
or
$ sudo service postfix reload

Edit "main.cf" settings:

$ sudo gedit /etc/postfix/main.cf

Specify:

myhostname = kctang.com.hk
# last line changed from server name to fully qualified domain name,
# otherwise some servers would not accept e-mails sent without fully qualified domain name, 2/4/2014

message_size_limit = 204800000
# last line added to increase the default 10 times, 5/4/2014
# increased to 20 times, 20/9/2018

maximal_queue_lifetime = 0
# last line added to report unsuccessful delivery immediately instead of after the default of 5 days, 12/4/2018

bounce_queue_lifetime = 0
# last line added, this should not be bigger than maximal_queue_lifetime, KCTang 20/5/2019

# smtpd_tls_mandatory_protocols = SSLv3, TLSv1 # commented, replaced below, KCTang 18/6/2021
smtpd_tls_received_header = yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache

# next 2 lines added to enhance security, KCTang 1/7/2021
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

# spf configuration, added but disabled, KCTang 1/6/2019
#check_policy_service unix:private/policyd-spf
#policyd-spf_time_limit = 3600s

# Milter configuration, added but disabled, KCTang 2/6/2019
#milter_default_action = accept
#milter_protocol = 6
#smtpd_milters = local:/opendkim/opendkim.sock
#non_smtpd_milters = $smtpd_milters
# end

#body_checks = regexp:/etc/postfix/body_checks
# last line added to refer to another file to check contents of email bodies, KCTang 29/9/2018, disabled after using amavis 1/6/2019

#header_checks = regexp:/etc/postfix/header_checks
# last line added to check headers, KCTang 30/9/2018, disabled after using amavis 1/6/2019

# virtual_alias_domains = kctang.com.hk
# last line added on 17/5/2019, but disabled since same domain name already defined as destination above, KCTang 20/5/2019

virtual_alias_maps = hash:/etc/postfix/virtual
# last line added to forward emails to another server, KCTang 17/5/2019

content_filter = smtp-amavis:[127.0.0.1]:10024
# last line added for amavis, KCTang 1/6/2019

Edit "master.cf" settings:

$ sudo gedit /etc/postfix/master.cf

Specify:

smtp      inet  n       -       y       -       -       smtpd
smtps     inet  n       -       y       -       -       smtpd
# last line uncommented
   -o smtpd_sasl_auth_enable=yes
# last line added to enable STARTTLS authentication
   -o smtpd_client_restrictions=permit_sasl_authenticated,reject
# last line added to reject if not authenticated, no space after ","
   -o smtpd_tls_wrappermode=yes
# last line added to force use of TLS
   -o milter_macro_daemon_name=ORIGINATING
# last line added

Create "body_checks" file, if required:

$ sudo gedit /etc/postfix/body_checks

Specify one or more lines of texts within //:

/unique text contained in email you do not want to receive/ DISCARD

"DISCARD" means delete from the server.

Create "header_checks" file, if required:

$ sudo gedit /etc/postfix/header_checks

Specify similarly.

(header checks added 7 May 2019)

Change Dovecot settings

Edit config file:

$ sudo gedit /etc/dovecot/conf.d/99-mail-stack-delivery.conf

Specify:

disable_plaintext_auth = yes

mail_location = maildir:~/Maildir:LAYOUT=fs
# LAYOUT=fs added to last line, to use "/" instead of "." to denote sub-folders

auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@

# IMAP configuration
protocol imap {
        mail_max_userip_connections = 1000
        # 10 in last line increased to 1000
        imap_client_workarounds = delay-newmail
}

# POP3 configuration
protocol pop3 {
        mail_max_userip_connections = 50
        # 10 in last line increased to 50
        pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}

# LDA configuration
protocol lda {
        postmaster_address = postmaster
        mail_plugins = sieve
        quota_full_tempfail = yes
        deliver_log_format = msgid=%m: %$
        rejection_reason = Your message to <%t> was automatically rejected:%n%r
}

# Plugins configuration
plugin {
        sieve=~/.dovecot.sieve
        sieve_dir=~/sieve
}

# Authentication configuration
auth_mechanisms = plain login

service auth {
  # Postfix smtp-auth
  unix_listener /var/spool/postfix/private/dovecot-auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

# The following are additional to those in 15-mailboxes.conf.
# They are to auto create Trash folders.
# Trash folders would not be backed up with back-in-time,
# and therefore would need to be re-created after email recovery is done from back-in-time.
namespace inbox {
  mailbox Trash {
    auto = subscribe
  }
}

# process_limit increased
service imap-login {
  process_limit = 200
}

(trash folder setting added 7 May 2019)

(full custom settings listed 5 Jan 2021)

Activate the changes:

$ sudo systmctl reload postfix
or
$ sudo service postfix reload
and
$ sudo systemctl restart dovecot
or
$ sudo service dovecot reload

Verify success

See whether the Postfix server is running:

$ telnet localhost 25

should display:

220 kctang.com.hk ESMTP Postfix (Ubuntu)

ehlo localhost

should display the following:

250-kctang.com.hk

250-PIPELINING

250-SIZE 102400000

250-VRFY

250-ETRN

250-STARTTLS

250-ENHANCEDSTATUSCODES

250-8BITMIME

250 DSN

Ctrl-]

to exit to "telnet >" prompt.

quit

to exit telnet.

Try also:

$ telnet localhost 995

similarly:

$ telnet localhost 465

should display either one:

Connected to localhost

Connected to kctang.com.hk

"Ctrl-]"

to exit to "telnet >" prompt.

quit

to exit telnet.

Specify internal email forwarding

Edit "aliases" file:

$ sudo gedit /etc/aliases

Specify:

postmaster: kctang
webmaster: kctang
kctcl: kctcl, kctclpop

meaning:

forwarding e-mails sent to postmaster@localhost and webmaster@localhost to kctang@localhost, no email will be left at postmaster or webmaster

(webmaster added 7 May 2019)

forwarding e-mails sent to kctcl@localhost to kctcl@localhost (itself) and to kctclpop@localhst, i.e. making a copy

"localhost" means "kctang.com.hk" in our case. For the email user name before "@", there is no need to create a file system user account for it if all emails addressed to it are forwarded elsewhere. The name serves as an alias only of the email account forwarded to.

(added 17 May 2019)

Activate setting everytime the "aliases" file has been changed:

$ sudo newaliases

Copy to external email accounts

(section added, 17 May 2019)

Execute:

$ sudo gedit /etc/postfix/main.cf

Specify at the end of the file:

virtual_alias_domains = kctang.com.hk
virtual_alias_maps = hash:/etc/postfix/virtual

Execute:

$ sudo gedit /etc/postfix/virtual

Specify to make a copy to itself and a copy to the external email account:

# from               to one or more addresses, separated by a space 
kctang@kctang.com.hk kctang@kctang.com.hk name1@external.account.name
kctcl@kctang.com.hk kctcl@kctang.com.hk name2@external.account.name

Omit making a copy to itself if only email forwarding is required.

Save and exit.

Execute after the "virtual" file is created or changed:

$ sudo postmap /etc/postfix/virtual

Execute:

$ sudo systemctl restart postfix

Test by sending emails.

Install anti-virus Clamav-Daemon

(section added, 30 May 2019)

Install clamav-daemon:

$ sudo apt update
$ sudo apt install clamav-daemon            (clamav-freshclam also automatically installed)
$ sudo dpkg-reconfigure clamav-daemon   (must be done, generally accept all defaults, set yes to scan emails)
$ sudo systemctl start clamav-freshclam
$ sudo systemctl start clamav-daemon
$ sudo systemctl status clamav-freshclam    (check if running)
$ sudo systemctl status clamav-daemon       (check if running, OK if reported "/bin/mkdir /run/clamav (code=exited, status=1/FAILURE" because directory already created)
$ tail -f /var/log/clamav/clamav.log        (see running progress, Ctrl-Z to exit)

(typo corrected, 8 May 2020)

Filter spam mails

(section added, 30 May 2019)

Execute to install various software:

$ sudo apt update
$ sudo apt install amavisd-new spamassassin
$ sudo apt install postfix-policyd-spf-python                                 (optionally required for spf) 
$ sudo apt install opendkim                                                   (optionally required for opendkim)
$ sudo apt install pyzor razor                                                (optional extras)
$ sudo apt install arj cabextract cpio lhasa nomarch pax rar unrar unzip zip  ("lhasa", not "lha")

Configure ClamAV:

$ sudo adduser clamav amavis
$ sudo adduser amavis clamav

Configure SpamAssassin:

$ sudo gedit /etc/default/spamassassin

~~Change "ENABLED=0" to:~~

ENABLED=1

(no longer set here, 8 May 2020)

Start the service:

$ sudo systemctl start spamassassin.service

Configure Amavisd-new:

$ sudo gedit /etc/amavis/conf.d/15-content_filter_mode

Specify:

use strict;

# Uncomment the two lines below to enable antivirus checking mode

@bypass_virus_checks_maps = (
   \%bypass_virus_checks, \@bypass_virus_checks_acl, \$bypass_virus_checks_re);

# Uncomment the two lines below to enable SPAM checking mode

@bypass_spam_checks_maps = (
   \%bypass_spam_checks, \@bypass_spam_checks_acl, \$bypass_spam_checks_re);

1;  # insure a defined return

Execute:

$ sudo gedit /etc/amavis/conf.d/20-debian_defaults

Change "D_PASS" to:

$final_spam_destiny = D_DISCARD

(deleted, 8 May 2020, reinstated, 26 Jul 2021)

Adjust the following values only if desired to flag more messages as spam:

$sa_tag_level_deflt = 2.0; # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 6.31; # add 'spam detected' headers at that level
$sa_kill_level_deflt = 6.31; # triggers spam evasive actions
$sa_dsn_cutoff_level = 10; # spam level beyond which a DSN is not sent

Execute:

$ sudo gedit /etc/amavis/conf.d/50-user

Specify:

$myhostname = 'kctang.com.hk';

Re-start the service:

$ sudo systemctl restart amavis.service

Edit the following file to specify domains to be whitelisted if necessary:

$ sudo gedit /etc/amavis/conf.d/40-policy_banks

Test that the Amavisd-new SMTP is listening:

telnet localhost 10024
Trying 127.0.0.1...
Connected to kctang.com.hk.
Escape character is '^]'.
220 [127.0.0.1] ESMTP amavisd-new service ready

Press Ctrl-] and enter "quit" to exit.

Configure Postfix master.cf:

$ sudo gedit /etc/postfix/master.cf

Add the following to the end of the file:

smtp-amavis     unix    -       -       -       -       2       smtp
    -o smtp_data_done_timeout=1200
    -o smtp_send_xforward_command=yes
    -o disable_dns_lookups=yes
    -o max_use=20

127.0.0.1:10025 inet    n       -       -       -       -       smtpd
    -o content_filter=
    -o local_recipient_maps=
    -o relay_recipient_maps=
    -o smtpd_restriction_classes=
    -o smtpd_delay_reject=no
    -o smtpd_client_restrictions=permit_mynetworks,reject
    -o smtpd_helo_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o smtpd_data_restrictions=reject_unauth_pipelining
    -o smtpd_end_of_data_restrictions=
    -o mynetworks=127.0.0.0/8
    -o smtpd_error_sleep_time=0
    -o smtpd_soft_error_limit=1001
    -o smtpd_hard_error_limit=1000
    -o smtpd_client_connection_count_limit=0
    -o smtpd_client_connection_rate_limit=0
    -o receive_override_options=no_header_body_checks,no_unknown_recipient_checks,no_milters

Add the following immediately below the "pickup" transport service:

     -o content_filter=
     -o receive_override_options=no_header_body_checks

Configure Postfix main.cf:

$ sudo gedit /etc/postfix/main.cf

Add the following to the end of the file:

content_filter = smtp-amavis:[127.0.0.1]:10024

Comment it with "#" at the start of the line if desired to stop using amavis.

Restart service:

$ sudo systemctl restart postfix.service

Check the hidden header of email received after the above to see the presence of one or more of the following:

X-Spam-Level: 
X-Virus-Scanned: Debian amavisd-new at kctang.com.hk
X-Spam-Status:
X-Spam-Level:

If present, the spam filter is working.

See https://help.ubuntu.com/lts/serverguide/mail-filtering.html.en for a full explanation of the above.

Delete filtered mails

(section added, 9 Dec 2020)

Filtered files are stored in /var/lib/amavis/virusmails and should be deleted regularly.

Edit to define a scheduled job to delete filtered files:

$ sudo crontab -e -u amavis

where:

-e = edit

-u amavis = user amavis

Define as follows:

0 0 * * * find /var/lib/amavis/virusmails/ -type f -mtime +15 -delete

which means at

0 = 0 minute

0 = 0 hour

* = every day of month

* = every month

* = every day of week

find /var/lib/amavis/virusmails = find in that directory

-type f = regular files

-mtime +15 = file modified more than 15 days ago

-delete = delete the file

End of Page

Note

Intro

Re-direct to server ports​

Install both Postfix and Dovecot​

Reconfigure Postfix

Change Dovecot settings

Verify success

Specify internal email forwarding

Copy to external email accounts

Install anti-virus Clamav-Daemon

Filter spam mails

Delete filtered mails

Re-direct to server ports

Install both Postfix and Dovecot