A mail delivery subsystem error is an error report sent by a mail server back to the sender of a message that was undeliverable. The sender of the error report will appear as "Mail Delivery Subsystem."
There may be a number of reasons why the email in question could not be delivered. The body of the message should contain an explanation. Common reasons include:
- User unknown
- Host unknown
- Mail quota exceeded
- Message exceeds maximum size
In some cases, a mail server will try to deliver the message repeatedly for a specified period, with five days being common, before returning it with an error report.
If a user receives several such error message a day, it may be a sign that the email account has been hacked. In some cases, error reports may be received even if the user has not sent any messages. The number of mail delivery subsystem error reports received may total in the thousands.
Since many servers recognize spammer mail servers and email addresses, spammers use the email addresses from the account they have hacked to send out their massive spam campaigns. When some of the spam messages cannot be delivered, the error report goes to the "from" address of the spam email.
The best thing to do if an account has been hacked is to change the password. In extreme cases where it does not stop, the email account may have to be closed and changed.