Hi,
@Martian & @dziobak
Actually the creation date of the message files on the server is correct. That is an imported message that was received on the 13th of January 2009 19:40:56 has as creation timestamp 2009-01-14 19:40:56.00000000. The ctime (time of last modification of the file info) however matches the time of the import. But strangely sorting by ctime on the filesystem doesn't sort the messages in the same order as RoundCube does, when sorting by index.
@Trizzunkz
It think it depends: If the mail index is a feature of the IMAP server, then sorting by the date column of the RoundCube messages database table, should perform better, at least from my feeling. If the index is of course maintained by RoundCube and based on the RoundCube database, then sorting by this index should/will perform better, but than it should be possible to fix this index by a relatively simple SQL-statement.
Regards
P.S.: Of course switching of the "Sort messages by index, when sorting by date" option solves the issue. I'm just wondering what this mysterious index is based on...