If one sends an e-mail to an invalid address (i.e. invalid1234)
then the rejection notice (i.e. User Unknown) is often returned to
sendmail with the 'return-path' blank.

Normally sendmail delivers this notice to the originator of the original e-
mail.

If linuxconf is used to generate the sendmail.cf, then about 80% of the
time the notification is discarded with a error message placed in the
maillog file.  (I assume that for the other 20% the return-path is
provided.  I know that the problem happens for all mail bounced from AOL,
but from lots of other places as well.)

I suspect the check_mail rule is the culprit, but I am not sure.

Work-around: Use /etc/sendmail.mc file and the m4 macros to generate your
sendmail.cf file.