One our mailservers was heavily loaded these days. Some (braindead, but actually it's not their fault) user sent a huge mail to a huge list of recipients, with many incorrect addresses.

Unfortunately, some of the receipients are users of Microsoft (apparently "Win 2003 Server SBS mit Exchange 2003"). Which somehow bounces back the mail to all original receipients. Some of which, well, do the same. My current count is somewhere at 1700 mails. Most were rejected by my mailserver, since I've quickly added a filter.

Search for "IMB Recipient" to find out why you better do not use Microsoft Small Business Server. Unless you want to become really unpopular with your people for flooding their mailboxes.

Microsoft Knowledge-Base description of the problem - now if that isn't a gross understatement for "your mailserver can be abused to mailbomb others".

Todays culprit: Engpass Marketing- und Services GmbH apparently can't configure their Microsoft mailserver properly.