Unix is a collection of operating systems that run on any hardware. Unix people rarely download executables except from very trusted sources.

Windows users will download executables very frequently and virtually all of them use a cpu that uses intel's instruction set.

True viruses are machine specific. So you can create a SunOS true virus or an HP-UX true virus. But a unix true virus cannot be written.

The first internet based attack was the morris worm which targeted several versions of unix simultaneously. It took down many unix based systems. It even crossed over to Milnet and took down hundreds of military systems. That was well over 10 years ago and unix security has improved a lot since then.

Unix tends to treat email as something for a human to read. It is very rare for a unix system to attempt to execute an incoming email message automatically. It also helps that there are other operating systems that are very vulnerable to viruses. They divert the attention of virus authors away from unix. Finally, unix still has several security problems of its own. A cracker who wants to breech unix security will target one of those.

These are the big factors that act to protect unix from viruses. True viruses that is....

However, people use the term virus loosely. I know the difference between a virus and a worm. However, I would not be amused if the virus scanning software that I bought for my laptop intentionally ignored worms.

And evil software doesn't always perfectly match the classical definitions. Those email viruses that target windows are not true viruses. But it's not clear what to call them... they are sort of a cross between a trojan horse and a worm or something.

Now consider your mail server: if an email virus targeted toward Windows arrives on your server, it will not magically disappear. Sure, it won't affect your unix based mail server. That doesn't mean that you want to forward it to your user's computer. I would call a unix mail server waiting to download windows viruses "infected". What other term would you use? (Actually, I kinda like the term "subclinical infection".)

So unix can be infected with anything and after you loosen up the definition of "virus" enough to include all evil software, unix can be affected by viruses.

As for antispam, click on home, then click on answers to frequently asked questions, then click on email antispam techniques and email filtering