Relay server messages to an external account with Postfix

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Software

apt update
apt upgrade
apt install postfix sasl2-bin mailutils

Mail server credentials

We need to define the credentials that will be used to establish the connection with the smtp server.

Create a file called sasl_passwd in /etc/postfix that contains the credentials. 

vi /etc/postfix/sasl_passwd 
[smtp.oamis.net]:587 account@example.com:8xxxxxx4

Hash the file 

postmap hash:/etc/postfix/sasl_passwd

Check the hash file has been created 

file /etc/postfix/sasl_passwd.db
/etc/postfix/sasl_passwd.db: Berkeley DB (Hash, version 9, native byte-order)

Change header FROM

Using smtp we can only send mail (FROM field in email header) as the user we are connecting with. 

vi /etc/postfix/sender_canonical
/.+/ account@example.com

Hash the file 

postmap hash:/etc/postfix/sender_canonical

Check the hash file has been created 

file /etc/postfix/sender_canonical.db
/etc/postfix/sender_canonical.db: Berkeley DB (Hash, version 9, native byte-order)

Configure postfix to be a relay

Edit /etc/postfix/main.cf and add/modify the following lines to our main.cf 

cat /etc/postfix/main.cf | grep -v "^#" | grep -v "^$"
smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
readme_directory = no
compatibility_level = 2
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = empcldgit01.reddog.microsoft.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = servername.example.com, $myhostname, servername, localhost.localdomain, localhost
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = loopback-only
inet_protocols = ipv4
relayhost = [smtp.oamis.net]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = may
sender_canonical_maps = regexp:/etc/postfix/sender_canonical
smtp_use_tls = yes

Restart Postfix

systemctl restart postfix
systemctl status postfix
● postfix.service - Postfix Mail Transport Agent
  Loaded: loaded (/lib/systemd/system/postfix.service; enabled; vendor preset: enabled)
  Active: active (exited) since Tue 2018-09-11 11:57:29 EDT; 7s ago
 Process: 3686 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
Main PID: 3686 (code=exited, status=0/SUCCESS)
Sep 11 11:57:29 empcldgit01 systemd[1]: Starting Postfix Mail Transport Agent...
Sep 11 11:57:29 empcldgit01 systemd[1]: Started Postfix Mail Transport Agent.

Send a test message

mail -s "test message" myemail@example.com <<EOF
> Hello
>
> EOF

Check the log file 

tail /var/log/mail.log

Add aliases for local accounts

List aliases in file /etc/aliases 

cat /etc/aliases
# See man 5 aliases for format
postmaster:    root
root:   account@example.com

Compile new aliases file 

newaliases

Reload postfix 

systemctl restart postfix

References

Daniel Simao 19:15, 11 August 2019 (EDT)

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