So we could see the origin of the error.
For instance, I was once bounced because someone used my server to send spam.
The error message received was:
Hi. This is the qmail-send program at smtp5.arnet.com.ar.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
18.104.22.168 does not like recipient.
Remote host said: 550 5.7.1 ... Mail from 22.214.171.124 refused: Vea http://abuso.cantv.net/bl/spam.html
Giving up on 126.96.36.199.
A clue for the bouncing reason can be find at the header of the bounced email. A number (5XX) followed by the rejection reason.
550 MAILBOX NOT FOUND
554 This mailbox is full
Take a look at the following explanation: Bonced email
Rejected or Bounced email will have an error code of 5XX where the XX is additional numbers, the point is the first digit is a 5. This indicates a permanent error condition. No amount of retrying will make the message go through so your mail server will return it to you. Rejected email means that the server did not even accept the message for processing. Bounced email means the message was received but is being sent back for one reason or another.
A delivery delayed notice will have an error code of 4XX instead of 5XX. These messages usually state that the server will keep trying for X number of hours or days.
[Edited at 2005-01-11 04:36]