Howto
Configure public DNS entries for mail server
Step 1
Configure an “A record” to be used for you server.
mail.multi.ucs A <your.pub.lic.ip>
Step 2
Configure a “PTR Record” for reverse resolution.
your.pub.lic.ip.in-addr.arpa PTR mail.multi.ucs
Step 3
Configure the MX Record. Use an A record! Do not use an IP address or a CNAME entry.
multi.ucs MX 10 mail.multi.ucs
Step 4
Configure Postfix to use the public domain name and accept Emails.
root@mail:~# ucr set mail/smtp/helo/name=mail.multi.ucs
root@mail:~# systemctl restart postfix
Step 5
Make sure your firewall accepts and forwards the needed ports.