Those who love Linux often cite the absence of viruses as one of the shining examples of why it is better than Windows. For the most part, that argument is valid and real, but a server administrator must look beyond his/her own system and consider others as well. Your server may not have spam or viruses, but it can still be the source of those problems if people are allowed to use your server to send them.
Tools such as MailScanner, SpamAssassin, and ClamAV can help minimize your server’s chances of being a virus carrier or spam hub. MailScanner will scan each email and use the other two tools: SpamAssassin and ClamAV, to determine if the email contains spam content or Windows virus attachments.
ClamAV is a free and open source Linux application that can be used to scan for viruses, primarily for Windows. Most Linux distribution repositories offer it for installation, and you can then keep the virus definitions updated by connecting to a virus tracking server.
SpamAssassin checks messages for spam and rates them according to their likelihood of being spam. If a message is over the threshold, the software will flag it, and the message will end up in the user’s spam folder. It takes a lot of work to train this software to fight spam effectively, and there are free pre-tested rules that you can download to make your spam fighting more efficient.
MailScanner is simply a scanner tool that relies on other technology to complete it. In runs in conjunction with the server’s mail transport system, such as Sendmail or Postfix, and makes sure only the messages you want to get through will actually get through. Like ClamAv and SpamAssassin, MailScanner is free and open source software. It supports numerous email servers, and virus scanning products.