Have you compacted the folders? Using the delete key marks the message for deletion and hides the message. Compacting the folder actually removed the messages and frees up the disk space.

Read about compacting here.

http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders

While you are at it you might read these tips about maintaining your email system.

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

that does not resolve the problem, once I was able to fix it but by erasing a hidden file, but I cannot remember how that was done.

If your mail folder does not contain more than 4Gb of mail then compacting will fix the problem. A few years ago we had to delete the files because a bug in the software prevented the compact working. That was cleared up long ago. Or are you still using Thunderbird V3

my problem is that it has reached the 4gb. I know there is a way to clear up that 4gb and still keep your emails in the folders. I have done it in the past but with many headaches and I cant remember how.

If you want to keep those messages but reduce the file size: Archive your Sent folder.

How to set up Archive options - if not already set.

• https://support.mozilla.org/en-US/kb/archived-messages

Archive by month and year and choose to 'keep exisitng folder structure of archived messages'

Then select the 'Sent' folder to see emails. click on first email to get focus. Press 'Ctrl' key and 'A' key to select all emails. They will be shown as 'conversations in the Message Pane'. click on the 'Archive' button.

This should empty the Sent folder and place all sent emails in 'Archives' under the respective year, month and Sent folder.