I set up a mail server using postfix and I'm having a weird problem. I read that you should put
myhostname = localhost
and all domains to virtual hosts. So did I and it's working for many email addresses I send emails to but quite some fail with following error:
<blah@example.com>: host example.com[195.XXX.XXX.XXX] said: 550
Invalid HELO string (localhost) (in reply to RCPT TO command)

Which is kinda understandable but when I put a FQDN as myhostname postfix is kinda screwed up, I get following error for incoming mails (I can still send though):
Aug 18 16:39:10 mailsrv postfix/local[5925]: 1D9E423E026F: to=<cojones@mydomain.foo>, relay=local, delay=3.9, delays=3.8/0.04/0/0.04, dsn=5.1.1, status=bounced (unknown user: "cojones")

Do you know why all of the sudden postfix wants to deliver the emails to local user accounts instead of the virtual ones? I only changed myhostname from "localhost" to the FQDN.

That's why. The default list of local domains (specified by mydestination) includes myhostname. If you just want to change the HELO name, you can use