Creative Spammers finding out valid accounts on my mail server!

[Adam Jackson]

Hi everybody,
I run a mail server that handles mail for multiple domains. Somehow, a spammer
is obtaining a list of valid mail accounts on my sendmail server and
addressing his spam messages to only those accounts. Many of these accounts
mail accounts have never been used by a person and are not posted around the
internet anywhere, which leads me to believe there is a way for an outsider to
query my mail server for a list of valid mail accounts.
Has anyone heard of this?

Thank you in advance!
Adam