a customer called in:
"we got many complains from our customers that the newsletter yesterday spread a worm (referring to a wellknown worm at that time). our logs state that it must have come from your server."
me: "well, i will of course check on that, but it seems to be impossible to me because we use a unix-server which is not affected by that worm."
caller: "no, we've got your IP here. the mails defenitely come from your server!"