A quick search reveals that the IMAP Quota is the amount of data that can be downloaded/stored from an e-mail program like Thunderbird. Once the IMAP Quota is reached, old e-mails will start being deleted. Try going to https://mail.google.com/mail/u/0/?pli=1#settings/fwdandpop and change the IMAP Folder Size Limit to none. If that doesn't work, you just have too many e-mails.

Where do you see 81%?

See 81% on status bar at the bottom of Thunderbird page. This gmail has hardly any emails in it - so I'm quite confused. Another gmail account I have is only 3%

What add-ons do you have installed?

Help (Alt-H) - Troubleshooting Information

Could you possibly post a screenshot of where the 81% is? I use Gmail in Thunderbird as well, and I don't see any % anywhere. I rarely delete e-mails in my trash folder, so I think I would at least see 1% somewhere. Also, how do you know it's the IMAP quota?

Here is screen shot

thanks

Those quota figures are notoriously inaccurate. Many servers simply lie

Go here https://www.google.com/settings/storage?hl=en at is your true storage situation. Note that google mail and google drive use that same available space.

Cheers Matt

Didn't realise it took into account Drive and +google pics

The shared storage catches me off guard too sometimes. I use Thunderbird on Windows 7, and I don't see that percent area in my window. Either way, glad you figured it out. :)