apparently-to list of addressees showing up in msg

[Jeff Welch <jwelch @ cutter . com>]

Has anyone had something like this show up in the outgoing list messages:

 Precedence: bulk
 Apparently-To: <jpsmith@company.com>
 Apparently-To: <johndoe@company.com>
 Apparently-To: <someone@company.com>
 Apparently-To: <someoneelse@company.com>

People seem to be receiving chunks of the list in the mail header! (I've
made up bogus addresses here, obviously).  What would cause this, and how
can I prevent it in the future?

Thanks for any information,

Jeff