What are the reasons for using ctime instead of mtime for
autopurge stuff in your webmail?
mtime can be manipulated by an IMAP client.
It can be some confusing for users, because when they log
into the webmail, then it moves all messages from new/
to cur/ directory and ctime of the files is changed,
of course. In the result, autopurging of some old files
in Trash folder can be delayed.
And some may consider it an added feature. If you don't care about the
trash, everything gets purged on time. If you open it, the most recent
trashed files get an extra lease on life.