Since I found Tim's description useful (though I haven't had the
problem myself), I'll add one:
The recipient SMTP server closes the connecton before sending a final
acknowledgement of having received it. The delivering server can't mark
it as delivered, so re-queues it, and the process continues.
Strangely, I've seen 2 addresses at the same domain with one exhibiting
the above behavior and one not.
On Fri, May 04, 2001 at 05:43:06PM -0400, Tim Pierce wrote:
> We occasionally get a complaint that someone has received dozens
> or even hundreds of copies of a single mailing list message.
> Each time we've been able to track down the source of the problem,
> The gateway opens an SMTP connection with the internal POP server
> and announces it has mail for 50 users. Suppose one of those users
> has a full mailbox. The POP server will return a 450 response for
> that user, meaning "can't deliver now, but try again later."
> The gateway *should* recognize that it should only try redelivering
> to that particular user. Not all systems do recognize this. Instead,
> they forge ahead and deliver the mail to the other 49 recipients, and
> then turn around and re-queue it for all 50. In a few minutes it
> redelivers the message to all 50, except for the one whose mailbox is
> full. So it re-queues the message again. And so on.