Note
- 1 Feb 2023: Correct typos.
- 10 Oct 2022: Add DKIM 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, enabled again to use dkim, KCTang 19/2/2022
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 993
("993", not "995", 1 Feb 2023)
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
Set up DKIM
(implemented, 19 Feb 2022
section added, 10 Oct 2022)
Set up DomainKeys Identified Mail (DKIM) to prevent email phishing by others by putting a digital key in the outgoing email headers so that the receiving email server can verify the digital key by looking up the public key in the DNS record of the sender domain.
Install OpenDKIM:
$ sudo apt install opendkim opendkim-tools
Add postfix user to opendkim group:
$ sudo gpasswd -a postfix opendkim
Configure:
$ sudo nano /etc/opendkim.conf
Specify:
# keep to log in /var/log/mail.log Syslog yes SyslogSuccess yes # change to "yes" if desired to log more details Logwhy no # keep Canonicalization relaxed/simple # uncomment Mode sv SubDomains no # keep OversignHeaders from # add AutoRestart yes AutoRestartRate 10/1M Background yes DNSTimeout 5 SignatureAlgorithm rsa-sha256 # keep UserID opendkim UMask 007 # comment #Socket local:/run/opendkim/opendkim.sock # add Socket local:/var/spool/postfix/opendkim/opendkim.sock # keep PidFile /run/opendkim/opendkim.pid TrustAnchorFile /usr/share/dns/root.key # keep KeyTable refile:/etc/opendkim/key.table SigningTable refile:/etc/opendkim/signing.table ExternalIgnoreList /etc/opendkim/trusted.hosts InternalHosts /etc/opendkim/trusted.hosts
Create a directory structure:
$ sudo mkdir /etc/opendkim $ sudo mkdir /etc/opendkim/keys $ sudo chown -R opendkim:opendkim /etc/opendkim $ sudo chmod go-rw /etc/opendkim/keys
Create the signing table:
$ sudo nano /etc/opendkim/signing.table
Specify:
*@kctang.com.hk default._domainkey.kctang.com.hk # if sub-domain used *@*.kctang.com.hk default._domainkey.kctang.com.hk
Create the key table:
$ sudo nano /etc/opendkim/key.table
Specify:
default._domainkey.kctang.com.hk kctang.com.hk:default:/etc/opendkim/keys/kctang.com.hk/default.private
Create the trusted hosts file:
$ sudo nano /etc/opendkim/trusted.hosts
Specify to sign but not to verify emails from localhost or the following domain:
127.0.0.1 localhost *.kctang.com.hk
("*" added, 1 Feb 2023)
Create separate folder for the domain and generate Private/Public Keypair:
$ sudo mkdir /etc/opendkim/keys/kctang.com.hk $ sudo opendkim-genkey -b 1024 -d kctang.com.hk -D /etc/opendkim/keys/kctang.com.hk -s default -v
Use -b 2048 instead of -b 1024 for bits of key if the domain name server permits.
-d for domain name
-D for directory to store the keys
-s for selector
The private key will be written to default.private file and the public key will be written to default.txt file.
Change owner and permission:
$ sudo chown opendkim:opendkim /etc/opendkim/keys/kctang.com.hk/default.private $ sudo chmod 600 /etc/opendkim/keys/kctang.com.hk/default.private
Display the public key:
$ sudo cat /etc/opendkim/keys/kctang.com.hk/default.txt
The string after the p parameter is the public key:
default._domainkey IN TXT ( "v=DKIM1; h=sha256; k=rsa; "
"p=..." ) ; ----- DKIM key default for kctang.com.hk
Define in the Domain Name Record on the domain name host:
Name = default._domainkey
Address or value = the text in ( ) above, but deleting double quotes and spaces
Type = SPF (txt)
Test DKIM Key:
$ sudo opendkim-testkey -d kctang.com.hk -s default -vvv
Successful if an opendkim-testkey output is key OK.
No problem if an opendkim-testkey output is key not secure because DNSSEC may not have been enabled on the domain name.
The DKIM record may take up to 24 hours to propagate to the Internet.
Can also go to https://www.dmarcanalyzer.com/dkim/dkim-check/, enter "kctang.com.hk" as the domain name and "default" as the selector to check DKIM record propagation.
In case of the query timed out error, comment out the following line in /etc/opendkim.conf file and restart opendkim.service.
TrustAnchorFile /usr/share/dns/root.key
Create a directory to hold the OpenDKIM socket file, and allow only opendkim user and postfix group to access it:
$ sudo mkdir /var/spool/postfix/opendkim $ sudo chown opendkim:postfix /var/spool/postfix/opendkim
Edit:
$ sudo nano /etc/default/opendkim
Comment out:
# SOCKET=local:$RUNDIR/opendkim.sock
Add:
SOCKET="local:/var/spool/postfix/opendkim/opendkim.sock"
Edit Postfix main.cf to configure Milter as shown above.
Restart opendkim and postfix service:
$ sudo systemctl restart opendkim postfix