I use Mozilla Thunderbird for desktop email. Today, I happened to notice that one of the messages in my Sent box was showing the wrong content when I clicked on the subject line. A bit of Googling later, and I determined that I had a corrupted index, which is simply remedied by deleting the "msf" file for that particular mailbox folder. This forces Thunderbird to rebuild the index. Sure enough, this fixed the problem.
All very interesting, I'm sure, but what was more interesting (bordering on the alarming in fact) was that I noticed that the hidden folder where the account data was located (AppData \ Roaming \ Thunderbird \ Profiles \ xxxxxxxx.default \ ImapMail) also contained all the data for every other IMAP account I've ever connected to, which in this case was over 6GB! So be warned; removing an account from Thunderbird leaves behind all the files and folders with all the locally cached data, just waiting to be read by who knows who or what!