I answered an email on a Yahoo! mailing list the other day. The individual was complaining that they got bounce messages from individuals, to whom they had not mailed, every time they mailed a message to the Yahoo! mailing list.
The real problem was that they really did not understand what Yahoo! was doing with their email.
If you subscribe to any Yahoo! mailing lists, or almost any other kind of mailing list, take a look at the From header in a list message.. You will see that Yahoo does not write the From header as From: Yahoo — they set the From header to show the email address of the person posting the message.
This is the normal practice for mailing lists. Of course, when an email bounces, you can guess what happens… e.g., who gets the bounce messages.
In this case, the person was getting bounces because a couple individuals have problems with their accounts, or they have canceled those accounts without bothering to unsubscribe from the Yahoo list. Or, their mailserver is refusing emails from Yahoo’s mailservers, or…
From one of the messages you quoted below,
> Final-Recipient: RFC822;[email address removed]
> Action: failed
> Status: 4.4.7
> Diagnostic-Code: SMTP; 452 try later
The above gives your key. The "secret" part is that many receiving mailservers will not specifically block an email, but they’ll send a message to the sending mailserver to "send it again later."
In some cases, they’re implementing a spam-filtering technique called Graylisting — they’ll note the email, where it came from, the destination, etc, and then allow the email to process normally when it arrives again. Of course, many spamservers will simply not try again.
In other cases, however, the receiving email server may have a problem (e.g., out of disk space) or may just use this as a method of refusing delivery.