Dovecot, getting error: Mail access for users with UID 222 not permitted

Solution 1:

Dovecot restricts the users that can login via the first_valid_uid setting -- the assumption is that system users (which should not be logging in to an IMAP server) all have UIDs below a certain threshold, and regular users (who should be the ones logging in to an IMAP server) have UIDs above a certain threshold.

So, first make sure you're not logging in as a system user (like postmaster). If you are logging in as a regular user, check your Dovecot config and make sure that the first_valid_uid setting accurately reflects your policies (or that of your distribution) for the UID range that represents regular users.