[cobalt-users] What is this Postmaster Notify email from the RaQ/Sendmail?

[spamcatcher <spamcatcher@xxxxxxxxx>]

Hello all,

This is a strange one, and I can't figure out why it was generated...

Got this email with the subject "Postmaster notify: User unknown" 
addressed to the postmaster at the webserver from the "Mail Delivery 
Subsystem <MAILER-DAEMON>."

Now, the user/email-address does not exist, have never existed, and have 
not been aliased. Checking the maillog, it seems the same relay have 
passed along email to this email address before (months ago) and it 
always get bounced without generating a notice to the postmaster.

Have not touched Sendmail in the past couple of months. Any ideas as to 
why a notice was generated this time?

Thanks!