Ashtonstle

The problem appears to randomly affect users (I’ve had it affect myself once too), and the only solution appears to one of (or a combination of) the options in the OP.

Check the status bar at the bottom of the screen to see if another process is running.

When this problem occurs, the message remains in your Outbox folder until you manually initiate a Send/Receive operation (for example, press F9 or select send or receive).

This article also contains resources for IT administrators to address reports about Active Directory Federation Services (AD FS) issues that are specific to Office for Windows platforms.

This extraordinary feat (when comparing the size of older PST and OST files to newer ones) also brought to the table more recent problems with the OST and PST Formats.