After switching my E-mail system from POP to IMAP, things seemed to working fine – until I found that messages were no longer being filtered and automatically redirected from the Inbox to a number of separate folders.
The problem was that the file, msgFilterRules.dat, that contains the filters is located in the local directory where all the mail folders are stored. When I created a new IMAP account, I renamed the old local directory and created a new one with the original name. Thunderbird then automatically created a new msgFilterRules.dat file in the new local directory. However, this new file was devoid of any rules and so none of my previously defined message filters was active.
The solution was to simply copy the original msgFilterRules.dat file to the new local directory and so overwrite the “empty” rules file with the original set of message filters. Once that had been done, new messages were duly sorted as desired.
Converting Thunderbird from POP to IMAP