thanks for sharing detailed addition to this solution!!

Symptom:
I noticed that messages, once sent, would go to the recipient, and would correctly be copied into the “Sent” folder ON THE SERVER at Mike’s email hosting provider. We could validate that by signing into the email hosting provider’s web interface and checking the status of the “Sent” folder – sure enough, a sent message would populate there just fine, and the timestamps would match up for when the message went through.
However, if I then opened Thunderbird and navigated to the “Sent” folder, it would time out looking at the contents: no messages after 5/9/2012 or so would be available. I noticed, however, that Thunderbird would continue to show me the “synchronizing” icon (a spinning circle) at the top of the Thunderbird interface.
I took a look at the network interface (after closing all other programs that might hit the network), and noticed that, while Thunderbird was attempting to sync the folder, it would hit the network about every 30 seconds. That’s a common timeout value for IMAP4, so that told me that IMAP4 was probably trying, but failing, to sync the folder.

Well, that suggested a solution. IMAP4 uses a Subscribe/Sync concept: it will only sync a local copy of data down from the authoritative copy on the mail server IF you ask it to “subscribe” to the folder.
My suspicion here was that the IMAP4 Sync with the “Sent” folder had been in the middle of a sync with the VERY large (1300+ messages) “Sent” folder on the mail server when an upgrade to Thunderbird went through and borked the local copy of the db: server data was fine, but local copy was corrupted.

So I forced a manual re-creation of the local db copy for the “Sent” folder by un-subscribing to the folder using IMAP4, restarting Thunderbird, and re-starting it.
Do this by (I’m working from memory, so details may alter):
In the main window of Thunderbird, right-click on the “inbox” folder.
On the context menu, choose “Subscribe”
In the dialog box that pops up, UN-check the “Sent” folder, then click “OK”.
Exit Thunderbird completely (not minimize, but File>Exit). This will force Thunderbird to write it’s db to disk, and will allow it to drop the “Sent” folder local replica.
Then re-open Thunderbird.
In the main window of Thunderbird, right-click on the “inbox” folder.
On the context menu, choose “Subscribe”
In the dialog box that pops up, RE-check the “Sent” folder, then click “OK”.
Exit Thunderbird completely (not minimize, but File>Exit). This will force Thunderbird to write it’s db to disk, and will allow it to drop the “Sent” folder local replica.
Re-Open Thunderbird. Let it catch up with replication – may take 5 minutes+ , so be patient. Clicking around like a madman now will only slow things down. Go get a cup of coffee, surf a little.
When you return to Thunderbird, navigate to the “Sent” folder. You should have headers for all the messages in the folder. When you click on a message, there may be a slight delay while Thunderbird fetches the actual message contents: this will be reflected in the status area in the lower left-hand portion of Thunderbird’s main window: pay attention to what you see there: it may give you further hints as to what’s going on if this didn’t resolve your problem.
NEXT: slim down your “Sent” items folder! Clear out the crud – delete old messages, move them to local folders that don’t replicate, etc. Remember, if you move messages to a purely local folder that doesn’t replicate with the mail server, you MUST be diligent with your own backups, because they’re no longer backed up by the server.

you’re welcome!

glad to know that :)