I’m writing these instructions up for posterity, since this is now the second time this incident has happened and I wanted to preserve the instructions for the MDBVU32 tool. The first time, it was a bad read receipt causing an annoying popup for every sent message; an outstanding client can’t send or receive any messages. (Original instructions are at howto-outlook.com.)

This situation can occur on most versions of Outlook and requires a repair utility from Microsoft to fix. To avoid completely trashing the mailstore, grab a copy of MDBVU32 (the Microsoft Exchange Server Information Store Viewer) from Microsoft or from here.

Start the utility, then click OK to the first dialog. Then,

• Click MDB > OpenMessageStore, select the item with the “D” next to it for the default PST file.
• The message will be in either MDB > Open Root Folder or MDB > Open IPM Outbox.
• Select the stuck message in the middle column and select lpMDB->AbortSubmit(), then Call Function.
• Select OK, then log out by MDB > Store Logoff > OK > OK
• Close the application by Session > Session Logoff, then close any remaining windows.