Copies of Sobig.E. Mailbounces from copies of Sobig.E sent with my email address on them. Notifications from virus scanners which were intelligent enough to detect Sobig.E but too stupid to realise that Sobig.E never has a correct return address on it. People who write to me saying "you wrote please see the attachment but you didn't send the attachment".
Spam. The particular kind of spam which arrives with a blank text-part and a base64 html-part, which gets through my procmail filter because the text part nixes both the "html only" and "entirely in base64" rules, and the base64 encoding makes keyword matching ineffective. Writing a rule that matches non-blank text parts may be "interesting". Possibly time to install a newer spam filter, but realistically that seems unlikely to happen in the near future.
Otherwise-intelligent staff members who write to support with their spam saying "is this new?" (as if we care) or with their mailbounces caused by spam or worms forged to appear to be from them, saying "I don't remember sending this email" or "is this evidence of something we should worry about?".
People who write to us saying "one of your users is sending spam" because they received a spam that had a From line of jhzyiidgw@comlab (I bet that user takes 5 attempts to log in every morning).