The usual remedy for this would be to advise you to compact the affected folder. But if it is so scrambled that it appears corrupted, there is a good chance that compacting will fall and may make things worse. First of all, back up your Thunderbird profile. https://support.mozilla.org/en-US/kb/profiles-tb Where are the messages that are being corrupted? Do you habitually leave a large number of messages in the Inbox? Whilst in theory Thunderbird should not have any problem with large folders, anecdotally it seems that those who allow messages to accumulate do suffer this type of corruption. Here is some old-school advice: http://kb.mozillazine.org/Keep_it_working_(Thunderbird) I am told that a lot of this is unnecessary, but it is how I use Thunderbird, and i don't get folder corruption.