There are worms out there (such as Nimda.E) that use Outlook address books
not just for lists of victims, but also as "From:" addresses. In other
words, your involvement might be having sent email to someone else who
is infected.
--Steve Bellovin, http://www.research.att.com/~smb
Full text of "Firewalls" book now at http://www.wilyhacker.com
An important addendum here - "having sent mail" includes posting to a mailing
list that has a subscriber. I've gotten a lot of complaints because the
actual perpetrator was a subscriber to NANOG or IETF or one of the many
SecurityFocus mailing lists I post to. And once you take the union of
*all* those lists, you start hitting the "birthday paradox" - it becomes
*very* likely that if you and the recipient know each other (by virtue
of being in the computer industry) that a third party has seen mail from
both of you.
Another way to look at it is that the "6 degrees" game can easily drop
2 or 3 degrees *really* fast if you allow "A and B both subscribe to the
same mailing list" as a connection.