1000's of Returned Mail messages every day!

Question

Hi All,
I am receiving thousands of Returned Mail and Undeliverable Mail messages every day in my Mail application.
Here is the problem description:
I used to have a MobileMe (.mac) account and a corresponding email address.  I did not renew my MobileMe account this year.  Now, whenever Mail receives any valid message to my existing RoadRunner email address, that same message is being automatically resent to the defunct .mac email address and since it is no longer a valid address, it is being returned to my inbox as Undeliverable or Returned.  The same message is being resent apparently many many times for each valid message I receive in my inbox.  The result is literally thousands of undeliverable email messages clogging my account.  Each time it is resent, it copies all the headers and the original message so by the time the same message is received over and over again the message sizes are well over 2 MB in size!  It is an endless repeating cycle.
What I have done so far:
Called RoadRunner - they said to delete and reinstall Mail
Deleted Mail and reinstalled
Deleted Mail preferences
Turned off all iSync functions
Searched for and deleted all references to the invalid .mac email address on my computer
Turned off all notifications from iCal
Upgraded OS from 10.5.x to 10.6.7
None of this has stopped the torrents of messages…PLEASE HELP.
Would not like to have to erase the hard drive and reinstall the OS from scratch if possible.
Here is what a typical returned message header looks like (Email address blocked out by me):
This message was created automatically by the mail system (ecelerity).
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
[email protected] (after RCPT TO): 550 5.1.1 unknown or illegal alias: [email protected]
Reporting-MTA: dns; hrndva-oedge04.mail.rr.com
Arrival-Date: Thu, 28 Apr 2011 13:25:39 +0000
Remote-MTA: dns; nk11b01-smtp-mx006.mac.com
Diagnostic-Code: smtp; 550 5.1.1 unknown or illegal alias: [email protected]
Last-Attempt-Date: Thu, 28 Apr 2011 13:25:39 +0000
Action: failed
Final-Recipient: rfc822; [email protected]
Status: 5.1.1
------ This is a copy of the original message, including all headers. ------
Return-Path: <[email protected]>
X-Authority-Analysis: v=1.1 cv=pN6kzQkhXdmdOr6Akjoh3kGBD/S3UyPMKQp53EJY+ro= c=1 sm=0 a=9Xqw66NJkoB9GertGQDpPQ==:17 a=n-kJSqksAAAA:8 a=nz4EaHi66Edgnas9-7sA:9 a=98jSFH7WqmUA:10 a=ayC55rCoAAAA:8 a=qyzg_sS0DHm6ZXvf9RAA:9 a=udDd3TAlAAAA:8 a=A-Ay9Xv3AAAA:8 a=13-m3CVyAAAA:8 a=Nq7WNfEEaPaVeQek9H8A:9 a=nCRpLGje-JdQZUhY4UwA:7 a=xKgkma0IJD8A:10 a=pdFTHh6Ffp0A:10 a=uqRLPZCTzCUA:10 a=60GRva4_FlB0_GV7:21 a=0a-loXuVl3_jPVwJ:21 a=9Xqw66NJkoB9GertGQDpPQ==:117
X-Cloudmark-Score: 0
X-Originating-IP: 50.90.215.242
From: <[email protected]>
Received: from [50.90.215.242] ([50.90.215.242:47870] helo=[10.0.1.4])
    by hrndva-oedge04.mail.rr.com (envelope-from <[email protected]>)
    (ecelerity 2.2.3.46 r()) with ESMTP
    id 73/88-28036-3CA69BD4; Thu, 28 Apr 2011 13:25:24 +0000
Content-Type: multipart/report; report-type=delivery-status; boundary="lvFcVbMq4Bmrb7Nos44Q+yDizFUn+uvxvchPCg=="
X-Mailer: Apple Mail (2.1084)
Date: Thu, 28 Apr 2011 13:24:24 +0000
Subject: Mail Delivery Failure
Resent-Date: Thu, 28 Apr 2011 09:25:23 -0400
Resent-From: Joe Blow <[email protected]>
To: [email protected]
Resent-To: [email protected]
Message-Id: <[email protected]>
--lvFcVbMq4Bmrb7Nos44Q+yDizFUn+uvxvchPCg==
Content-Type: text/plain
This message was created automatically by the mail system (ecelerity).
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
[email protected] (after RCPT TO): 550 5.1.1 unknown or illegal alias: [email protected]
THANK YOU FOR READING AND HELPING IF YOU CAN.
Bern

Answer 1

Are the returned messages showing all different recipients? If so, it's possible someone has hacked into your MobileMe account and is using it to send out spam.
A brute force spamming will result in thousands of failed return messages. That happens when a spammer sends out mail to a know mail server, such as Hot Mail. They'll send the same message to [email protected], [email protected], etc. Do this for thousands of addresses and many will be returned by the receiving server as undeliverable because there is no user on their system by [email protected], and any other address with no match.
Or, your address is simply being spoofed by a prolific spammer. They picked up your email address as a valid address and are spoofing it in the From field. Doesn't matter that the thousands of emails that went out from them with your name as the sender didn't come from you, the receiving server just assumes it did and you get the bounce messages.
For the first, go in and change your password and see if that stops the flow. For the latter, there's really nothing you can do. Spammers will only use someone's address for a short while. Otherwise, the "from" address gets blacklisted automatically by mail servers, which would cut off the spammer's ability to send out more crap in your name. Then it's someone else who has to put up with it for a few days.