get a latter version... seriously.

I recall there being some sort of bug that was fixed that did exactly what you show... I think it was gmail specific. But my memory suffers bitrot today.

This is the newest version applicable for my OS, which is Ubuntu 10.??

I seriously would not use any Ubuntu again, but for now I am stuck with this outdated version,because some software that I need does not run on newer Linux. (everything else work fine even years after EOL, and I do not fear for security holes.)

So, seriously, thx for your advise,, but I would appreciate more some hint that enables me to fix that flaw. that btw. only occurs in one of my really heavy loaded IMAP accounts. The others are good, even from the same provider, which is not gmail or some other main stream data leech.

I really need to fix this issue with this version.

Is there a way to rebuild the whole repository for that account?

Your not stuck with an old Thunderbird :) http://askubuntu.com/questions/50931/how-do-i-install-the-latest-stable-version-of-thunderbird

With IMAP mail accounts, you can simply remove the account and add it again. The process will however create new folder in the imapMail folder of your profile. Manual deletion of the old mail containing folder would be a good idea if significant amounts of space are to be released.

is your server is XXX.business.com and the old folders will be XXX.business.com and each subsequent add will append a number such as XXX.business.com1 to the folder created in the file system.

Note that you should also remove the associated entry in outgoing server (SMTp ) as the new add will try and recreate it often with the same name and poor results (failure)