Please Note: Despite that it might still work, this project will not be updated anymore and is here just for reference.
IMAP authentication for Wordpress 4.0.0+
This plugins replaces the regular authentication of wordpress and offers instead the authentication of users via IMAP accounts. Password reset and retrieving are suppressed in order to avoid confussion.
The IMAP authentication server as well as an automatic padding of the domain to a user name can be set on the plugins setting page under Settings > IMAP Authentication. Furthermore, users having a IMAP account on the defined server can automatically be register if set so.