Spam occasionally includes requests for read receipts. In some cases this is so the spammer can check for valid email addresses but most of the time it’s not intentional or the spammer just wants to be doubly annoying since the sender’s address is invalid and the read/not read receipt bounces back to the victim in the form of an NDR.

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.

I haven't been able to repro it using my IMAP server, but if it happens to you, mark the Junk E-mail messages read before closing Outlook or before syncing with the IMAP server. This will trigger the receipt request and Outlook will respond according to your tracking setting. You can also avoid problems by only emptying the Junk folder using Outlook (marking the folder read first just to safe).