The majority of viruses are propagated via email. While it is still possible to receive viruses from floppy disks and CD-ROMs, or from Internet worms and rootkits that directly attack vulnerabilities in your operating system, you are much more likely to receive viruses via email – often by inadvertently opening seemingly innocent attachments. Of course it is important to have an up-to-date virus checker on your desktop, but defense in depth is the best approach, and this is why you should be protected from spam and viruses at your receiving SMTP server. Virus protection at the mail server prevents undesirable email from reaching your desktop, and it also keeps viruses out of your POP3 or IMAP account. Filtering from the mail server reduces the load on your desktop PC or laptop, reduces the amount of bandwidth you will use, and greatly reduces the risk of spam and viruses reaching your desktop. An additional advantage of providing this extra layer of protection is that, by using a virus checker on your desktop that is different from the content checker on your SMTP server, you reduce the risk of a virus passing undetected.

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