Sure, there are many alternatives to BitDefender.
- Barracuda makes a drop-in appliance which is used by many smaller companies. I'm not a big fan, but it works well enough.
- OpenBSD has a the very nice SpamD daemon specifically for greylisting and lightweight filtering in front of 'heavy' techniques such as SpamAssassin or Bayesian filtering. (SpamD doesn't do antivirus on its own.)
- Google has a hosted service which is super-simple to set up (provided you can change your domain DNS MX records), and can both do spamfiltering, antivirus and email archiving.
For all of the above there are several competing products / similar solutions from others.
Personally, I prefer light-weight greylisting in front of an individual Bayesian spam-filter, as this solution in my experience gives the lowest false positive rate. This does however require some training of the end users. If that's not an option, then I would probably go for a hosted 'set it and forget it' solution next, such as Google's Postini service.