Outlook is sending unknown messages

This seems to be most common with IMAP accounts. In many cases the messages are undeliverable and are returned to your mailbox as an NDR.

Cause

Newer versions of Outlook offer the ability to control if read receipts are sent but there may be a bug with IMAP accounts and read receipts in Outlook 2007. When you move messages with a receipt request to the Junk E-mail folder and empty the Junk E-mail folder using another client, the setting in Tools, Options, Email Options, Tracking Options may be ignored. If so, read receipts are returned when Outlook syncs with the server and purges the junk folder.

The steps to repro are as follows:

1. Send yourself some messages with receipts requested and move them to Junk E-mail.
2. Sync with IMAP and update all folders then close Outlook.
3. Empty junk email folder using a different client.
4. Reopen outlook.
5. When outlook updates the folder, it ignores the Tracking setting and always returns receipts as it syncs the junk mail folder, purging the messages deleted from the other client.

Workaround

Until Microsoft releases a fix, do the following:

1. Always mark messages as read before emptying the Junk Email folder.
2. Always empty the Junk Email folder from Outlook.

If you don't ever want to send receipts back, set Outlook to never ask and never send, otherwise set Outlook to always ask before sending. (Tracking settings are found in Tools, Options, Email Options, Tracking Options.)

If you move message between folders, this can also trigger a Not Read receipt, since moving messages marks the message for deletion in the original folder.

Sample Read Receipt

To: [an address you don't recognize]
Subject: Not read: [Varying Subjects]
Body:
Your message

To: [your address]
Subject: [Varying Subjects]
Sent: 9/25/2008 4:19 AM

was deleted without being read on 10/26/2008 9:21 AM.

The only difference between Read and Not Read receipts is the subject is prefixed with 'read', instead of 'not read' and the text says it was "read on [date]".