I've been seeing a strange issue with Thunderbird IMAP in combination
with MSU Mail. Their Sent messages are split into folders by month,
without the user doing anything. I can't find an option in Thunderbird
or mail.msu.edu that would explain it, and this is happened to two
people so far. One day all their Sent messages where in one folder, and
the next day, they had moved into folders according to month underneath
the Sent folder.