Wasn't sure where to put this but since I have a total control server thought it best in here.

Basically I have people using my mail system for spam or spoofing.

mydomain.com or mycustomerdomain.com sending me virus e-mails, also thousands of spam daily which I can't get rid off.

Unfortunately you can not prevent other people from sending email with your domains as the return address. The best you can do is set up SPF or Domainkeys and help remote systems determine the legitimacy of the mail they receive.

Now if your server is actively relaying spam, that's another matter.

My server is not actively relaying spam as far as I'm aware, the spam just appears to come from addresses which don't actually exist. I have limited its amount after tweaking some settings but its still a large amount.

Yep. I get that too. Anyone can forge anyone's FROM: address. If you're using "catch-all" email accounts, you'll want to disable them.

Meaning, I get a lot of bounces from spam campaigns that claim to be from xyzyzxy@mydomain.com. Obviously there's no one named xyzyzxy at my mail server, so now I just reject them. I liked having a catch-all so customers could reach me if they typoed "mat@mydomain.com" instead of "matt@mydomain.com", but it just became impractical to maintain. I was losing real emails in a sea of spam anyway.