How can I make several groups of users and tune dovecot in a such way, that one group can use plain text auth (for example), second - APOP, third - MD5-CRYPT, etc?
Thanks in advance.
I don't know if it can be done because of the IMAP/POP login process.
When you start (for instance) an IMAP session, the server proposes you a series of authorization mechanisms. The client chooses one of them (possibly the best) during the login process, but is the client that makes the cohice.
You cannot force the server to present a limited set of authorization mechanisms BEFORE the user logs on.