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CHAMP0Rick, my guess is that it has hit the magical 4Gb maximum size.
Copy/Move everything in Sent to a local folder
right click the folder and select compact (Note this does not work if the folder view is 'unified')
Search your computer for files named sent. Is there one at or very close to 4Gb in size?
If yes. Rename it to send-old
Hopefully everything is good.
see the change log