Let’s say you have a user who can’t receive e-mail. Each time they send a message to the server, this pops up in the mail logs:

postfix/smtpd[23897]: NOQUEUE: reject: RCPT from remotemailserver.com[]: 554 <user@domain.com>: Relay access denied; from=<user@otherdomain.com> to=<user@domain.com> proto=ESMTP helo=<remotemailserver.com>

This is happening because Postfix is receiving e-mail for a domain for which it doesn’t expect to handle mail. Add the domains to the mydestination parameter in /etc/postfix/main.cf:

mydestination = domain.com, domain2.com, domain3.com

If you have a lot of domains to add, create a mydomains hash file and change the mydestination parameter:

mydestination = hash:/etc/postfix/mydomains

Create /etc/postfix/mydomains:

localhost               OK
localmailserver.com     OK
domain.com              OK

Then run:

# postmap /etc/postfix/mydomains

This will create the hash file (mydomains.db) within /etc/postfix. If you’ve just added the directive to the main.cf, run postfix reload. However, if the directive was already there, but you just adjusted the mydomains and ran postmap, then there is nothing left to do.