Scam-backscatter 1.2.2

Scam-backscatter prevents backscatter (accept and bounce) on mail servers which dont host mailboxes locally. The project validates mailboxes by verifying the recipient addresses hosted on a different mail server.

Backscatter is when a mail server accepts an email with a forged sender address, then generates a bounce (non-delivery receipt) to the sending address. The figure below shows an example where the sender address is forged:

1. An email is sent by smtp.example.com with a forged sender address of user1@example.org to user2@example.net.

2. The email is accepted for delivery by smtp.example.net.

3. The email is then forwarded to mailbox.example.net.

As there is no user2 mailbox on mailbox.example.net, a bounce message (non-delivery receipt) is generated by mailbox.example.net to notify the sender address (user1@example.org) of the delivery failure.

4. The bounce message is sent to the mail server at smtp.example.org.

5. user1@example.org receives the bounce message.

Your mail server can prevent backscatter by validating the recipient email address in step 2.