POPcon says that it drops copies of downloaded email - what causes this message?

Q:

-----Original Message-----

Date: Mon, 1 Aug 2005 16:56
Subject: POPcon dropping mail

Gday Claus,

I get this error in POPcon log. What is it? And how can I avoid it?

      same Message ID [long string of characters] already sent to localuser@mydomain.com - dropping this copy

A:

Hi,

this is not an error - POPcon keeps duplicate emails from happening by comparing the recipient address and message ID to a list of emails that it already transfered to your exchange server. If it already did transfer this exact message to this exact mail address it will drop this copy and not forward it again.

Duplicates often happen when somebody sends a mail to two (or more) users in your domain and you are using a domain-wide catch-all POP3 mailbox. The sending program does not know that both recipients actually share the same POP3 mailbox so it sends identical copies of the mail to the two recipients. Both copies end up in the same mailbox from where they are downloaded by POPcon. When POPcon downloads the first copy the mail headers are checked and both recipients are found in the TO: or CC: headers and POPcon forwards this first copy to both recipients. When the second copy arrives both recipients already received a copy and POPcon drops the second copy in order to keep duplicates from happening.

Best Regards,

Claus Christensen