A large number of email messages (NOT enough to affect the server's performance but enough to be a major pain) with the subjectUndelivered Mail Returned to Sender
The recurring pattern is a legitimate email account with 2 or 3 letters added to the UID part or the email address (firstname.lastname@example.org).This is the mail system at host server.domain.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
Is this an attack on the server (spam or otherwise)
or something else?