Install and Configure SMTP Server

root@mail:~# apt -y install postfix sasl2-bin # on this example, proceed to select [No Configuration] # because configure all manually +------+ Postfix Configuration +-------+ | General type of mail configuration: | | | | No configuration | | Internet Site | | Internet with smarthost | | Satellite system | | Local only | | | | | | <Ok> <Cancel> | | | +--------------------------------------+ root@mail:~# cp /usr/share/postfix/main.cf.dist /etc/postfix/main.cf root@mail:~# vi /etc/postfix/main.cf # line 82 : uncomment mail_owner = postfix # line 98 : uncomment and specify hostname myhostname = mail.srv.world # line 106 : uncomment and specify domainname mydomain = srv.world # line 127 : uncomment myorigin = $mydomain # line 141 : uncomment inet_interfaces = all # line 189 : uncomment mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain # line 232 : uncomment local_recipient_maps = unix:passwd.byname $alias_maps # line 277 : uncomment mynetworks_style = subnet # line 294 : add your local network mynetworks = 127.0.0.0/8, 10.0.0.0/24 # line 416 : uncomment alias_maps = hash:/etc/aliases # line 427 : uncomment alias_database = hash:/etc/aliases # line 449 : uncomment home_mailbox = Maildir/ # line 585: comment out and add #smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU) smtpd_banner = $myhostname ESMTP # line 659 : add sendmail_path = /usr/sbin/postfix # line 664 : add newaliases_path = /usr/bin/newaliases # line 669 : add mailq_path = /usr/bin/mailq # line 675 : add setgid_group = postdrop # line 679 : comment out #html_directory = # line 683 : comment out #manpage_directory = # line 688 : comment out #sample_directory = # line 692 : comment out #readme_directory = # line 692 : if also listen IPv6, change to [all] inet_protocols = ipv4 # add follows to last line # disable SMTP VRFY command disable_vrfy_command = yes # require HELO command to sender hosts smtpd_helo_required = yes # limit an email size # example below means 10M bytes limit message_size_limit = 10240000 # SMTP-Auth settings smtpd_sasl_type = dovecot smtpd_sasl_path = private/auth smtpd_sasl_auth_enable = yes smtpd_sasl_security_options = noanonymous smtpd_sasl_local_domain = $myhostname smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination root@mail:~# newaliases root@mail:~# systemctl restart postfix

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Configure additional settings for Postfix if you need. It's possible to reject many spam emails with the settings below. However, you should consider to apply the settings, because sometimes normal emails are also rejected with them. Especially, there are SMTP servers that forward lookup and reverse lookup of their hostnames on DNS do not match even if they are not spammers.

root@mail:~# vi /etc/postfix/main.cf # add to the end # reject unknown clients that forward lookup and reverse lookup of their hostnames on DNS do not match smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname, permit # rejects senders that domain name set in FROM are not registered in DNS or # not registered with FQDN smtpd_sender_restrictions = permit_mynetworks, reject_unknown_sender_domain, reject_non_fqdn_sender # reject hosts that domain name set in FROM are not registered in DNS or # not registered with FQDN when your SMTP server receives HELO command smtpd_helo_restrictions = permit_mynetworks, reject_unknown_hostname, reject_non_fqdn_hostname, reject_invalid_hostname, permit root@mail:~# systemctl restart postfix