The meaning of BOUNCE or DSN

A Bounce or DSN (Delivery Status Notification), NDR (Non-Delivery Report/Receipt), or NDN (Non-Delivery Notification), or by abuse of language: Mailer-Daemon, is an e-mail sent automatically by the postmaster to the senders. In general, this type of e-mail appears in recipients’ mailboxes in two circumstances: when the destination mail servers have failed, or when the recipient’s e-mail address is incorrect or erroneous. The DSN is usually written in English, depending on the configuration of the original recipient’s mail server.

Understanding the Bounce received by users
When sending an e-mail, the sender can expect to receive two kinds of Bounce : one indicating that the e-mail has been rejected for various reasons, and one indicating that it has been sent successfully. DSNs are sent by the postmaster or without sender: “MAIL FROM :<> “.

If the recipient’s address is incorrect, i.e. the user name before the arobase is wrong or does not exist, a 550 MAILBOX NOT FOUND User Unknown will be sent. On the other hand, a 550 MAILBOX NOT FOUND Host unknown will take its place if the error is in the domain name after the arobase.

If the recipient’s inbox is full, a 552 MAILBOX FULL or 550 Quota overflow will be sent to the sender.

Sometimes, recipients set up filtering parameters on their e-mail inboxes to prevent the invasion of spam and junk mail. If your e-mail has been stopped by this filter, you will receive a message like 550 User name AOL IS NOT ACCEPTING MAIL FROM THIS SENDER.

When sending large file attachments, decompression software is sometimes required. If your recipient’s inbox rejects your e-mail because of its size, a bounce containing text close to : 552 MESSAGE EXCEED MAXIMUM FIXED SIZE will be sent to you by the postmaster.

Faced with an inbox blocked by countless unread e-mails, your e-mail will no longer be received, and in return you’ll receive a 450 MAILBOX BUSY to let you know. These return codes starting with “45x” may also correspond to a problem with the mail server (a temporary outage). This type of return code is also used for greylisting.

If the e-mail message reaches the sender’s server successfully, a DSN Successful Mail Delivery Report will be sent to you. Most of the major email consumers have a dedicated website with detailed explanations of their error codes. For example:

http://postmaster.free.fr/
http://help.yahoo.com/l/fr/yahoo/mail/postmaster/
http://postmaster.aol.com/
http://postmaster.facebook.com/
http://mail.live.com/mail/postmaster.aspx
http://assistance.sfr.fr/
http://assistance.orange.fr/

Bounce as spam
The arrival of a Bounce in your inbox can also be a source of annoyance, especially if you haven’t sent any e-mail from your outbox. This may be because a malicious person has fraudulently used your address to send an e-mail, or because the sending is caused by a virus. Receiving several Bounces at the same time may mean that your computer is being used as a mailbomber without your knowledge. Spammers can send several spam messages with attachments from your machine to all the addresses they can collect in your address book, network neighborhood, etc. Although ALTOSPAM is able to distinguish between a real bounce and a bounce actually originating from e-mail addresses usurped by a third party, we recommend that in the event of an attack of this type, you set altospam to : “Refuse all bounces”, in the configuration section.