You have a few choices to make
Note that DMA is like Sendmail. You absolutely need to define an FQDN somewhere. It is fine if you just tune your /etc/hosts file enough, while using a short hostname, though. Beware that you need an FQDN for your cluster names even (in case it is the same name as for the public network).
#cluster network x.x.x.x FQDN SHORT #public x.x.x.x FQDN SHORT
#netbsd/slackware cp -i /etc/mail/aliases /etc/mail/aliases.dist vi /etc/mail/aliases #debian/ubuntu cp -i /etc/aliases /etc/aliases.dist vi /etc/aliases root: REAL-EMAIL-ADMIN newaliases #postalias /etc/mail/aliases
#/usr/share/postfix/main.cf.debian
mv -i /etc/postfix/main.cf /etc/postfix/main.cf.dist
sed '/^[[:space:]]*$/d; /^[[:space:]]*#/d' /etc/postfix/main.cf.dist > /etc/postfix/main.cf
there’s nothing to change, Ubuntu/focal defaults are fine already
grep ^alias /etc/postfix/main.cf
grep ^mydestination /etc/postfix/main.cf
NO NEED TO LISTEN ON INET/INET6 (unix socket is enough)
mv -i /etc/postfix/master.cf /etc/postfix/master.cf.dist sed '/^[[:space:]]*$/d; /^[[:space:]]*#/d' /etc/postfix/master.cf.dist > /etc/postfix/master.cf vi /etc/postfix/master.cf #smtp inet n - y - - smtpd
can you reach your relay?
smarthost=x.x.x.x #nmap -p 25,465,587 $smarthost nc -vz $smarthost 25 465 587
now define it
#/usr/share/postfix/main.cf.debian mv -i /etc/postfix/main.cf /etc/postfix/main.cf.dist sed '/^[[:space:]]*$/d; /^[[:space:]]*#/d' /etc/postfix/main.cf.dist > /etc/postfix/main.cf vi /etc/postfix/main.cf myorigin = $mydomain relayhost = x.x.x.x grep ^alias /etc/postfix/main.cf
postfix check && echo OK || echo FAIL systemctl restart postfix ls -lF /etc/postfix/makedefs.out rm -f /etc/postfix/makedefs.out
tail -F /var/log/maillog tail -F /var/log/mail.log date | mail -s `hostname` root
Postfix https://calomel.org/postfix.html
Chapter 27. Mail and news https://www.netbsd.org/docs/guide/en/chap-mail.html