BleepingComputer.com: Spammers Use New Technique To Evade Filters

Spammers have stepped up efforts to use encrypted attachments to evade filtering systems...The technique relies on the fact that many spam systems can't scan inside emails containing encrypted or password-protected attachment, and work out that they are not legitimate. Without a rule to block such attachments, most systems will pass on the email to recipients, handing spammers an important victory in the battle to get spam through...

...malware writers are adopting social engineering techniques common among spammers to lure victims into opening attachments...