As a result of the Outlook to PST migration I posted about recently, the Outlook IMAP bug reared its ugly head.
IMAP is great for a number of reasons, one of which is that the email store can be accessed from multiple clients.
User's often configure their email clients to never send "read receipts". Who doesn't? Personally I find requests for read receipts invasive and rude so I never use them or reply to them.
Unfortunately for those who agree with that sentiment and use Outlook with IMAP, there is an Outlook bug which under certain circumstances will cause Outlook to do just the opposite of what is desired and send out "not read" responses, without even prompting for permission.
In my experience using Outlook, readPST, and Thunderbird to migrate Outlook emails to IMAP, messages which had previously been read were marked as unread when uploaded to the imap server. If Outlook saw them as unread and they had a read receipt request, and then another client would mark the message as read and then delete it or move it, Outlook would send a "not read" response to the sender.
Since I was migrating tens of thousands of emails, Outlook was trying to send over a thousand messages before it was noticed. I took two measures - I marked additional messages as read before uploading to the IMAP server, and then as suggested on the Outlook bug report thread by Monaghans, I setup a Postfix header check to intercept the errant notices:
/^Subject:.Not read:/ REDIRECT email@example.com
Subject: Not read: blahblahblah Date: Sat, 2 Jan 2010 14:06:13 -0500 Message-ID: <004201ca8bde$d160c0e0$742242a0$@com> MIME-Version: 1.0 Content-Type: application/ms-tnef; name="winmail.dat" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="winmail.dat" X-Priority: 1 (Highest) X-MSMail-Priority: High X-Mailer: Microsoft Office Outlook 12.0 Thread-Index: Acjmqn0MfZuP9DwfQRKVovz4Y9Pde2lNBqrA X-MS-TNEF-Correlator: 000000001043424A784A894E8FBC74DB2524AEDE84CB4200 Importance: High