According to the article here: https://support.mozilla.org/en-US/kb/compacting-folders you will see this error message if a folder exceeds 4 GB. It's telling you that your Inbox is the problem folder, so emptying or deleting other folders will not solve the problem. The article is worth reading anyway so that you understand what's going on. I can recommend this article too: http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29 It recommends keeping the Inbox ''empty'': as soon as you read incoming mails, move them into other folders -- 'response needed', 'action needed' and so on -- and try to keep all the folders fairly small.