Hey,
Postfix’s myhostname variable isn’t that important in this context. What actually is important, is the name Postfix sends as part of the HELO…/EHLO… SMTP command. That host name must be resolvable to an IP address.
Luckily Postfix has a separate variable for that: smtp_helo_name. And there is a UCR variable for that: mail/smtp/helo/name. It defaults to the server’s (most likely internal) FQDN, but can be changed to a name that is publicly resolvable.
Don’t touch myhostname. There’s no need for it. But do make sure that smtp_helo_name is publicly resolvable.
Kind regards,
mosu