Server hostname appearing on email

I seem to remember that Joe said that we have to have a fully qualified domain name (FQDN) as our server hostname. Is this right?

Example:
Server hostname: www.domainname1.com
Domain name hosted on server: domainname2.com
Domain name hosted on server: domainname3.com

The problem I have is, when I send an email (e.g. me@domainname2.com) via Usernmin, the message header contains the hostname (domainname1.com). Incidentally, the hostname is also an active website. I don't really want people to associate domainname2.com with domainname1.com

What can I do to remove the hostname from email headers?

Many thanks.

Status: 
Closed (fixed)