It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup done prior to the corruption.

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Okay that is helpful. I understand the solution suggested and would easily be able to do that on my own up to date version of Thunderbird, however unfortunately, the computer that has the corrupt sent folder is an old out of date Vista computer with an older version of Thunderbird! Thunderbird is requiring a Vista update prior to updating Thunderbird which I am hesitant to do because I don't want to create new conflicts with other programs that are running fine!! Any other suggestion as to how to fix the corrupt .msf file within the sent folder. I had a tech do that once before over the phone support.

The steps suggested do not require a Thunderbird update.

In the older version I don't see where to push "Repair Folder" It is not under properties when I right click on the folder like it is in my newer version of thunderbird.

Thank you for you help! amy

Believe me I hoped it was that simple, Repair Folder, but I don't think the older version has this feature.

Close Thunderbird, and delete the corresponding .msf file. Or update to the latest version.

I did it and it is working perfectly! Can't believe it! Yay!! I was able to fix something for my co-worker not even my own problem! Thanks for all your help-Think this is the end of this question. Amy