I am at last planning to move from POP3 to IMAP. I use maildir format to store mails and I want, at least for a short time, to keep a fallback route.
I'm trying without success to find out how IMAP organizes and stores mails. As far as I can see I can continue to use my current MTA to prefilter and store mails in maildir structure. I can configure an IMAP server to operate with that structure.
The core of my question is, does an IMAP server leave mails in the maildir folder and simply create indexes to set up virtual user folders, while the [maildir]/new/ folder expands indefinitely? Or does it move/copy mails to its own physical storage - so that e.g. they can mirror the user's folder structure better and avoid possible file system inefficiencies from having all mails in one huge folder?