I wrote a plugin for phpBB3 to authenticate users through anything supported by the PHP IMAP functions (which includes the “IMAP protocol, as well as the NNTP, POP3 and local mailbox access methods”).  For this to work you need to have PHP configured to load the imap extensions.  The plugin supports authenticating users from two sources (a default source and a secondary source which is tried on failure of the first one).
Grab auth_imap.txt and upload it to your phpBB3 installation directory as  includes/auth/auth_imap.php
Add to the bottom of  language/en/acp/board.php (right before the ?>) the following block:
$lang = array_merge($lang, array(
‘IMAP_SERVER’           => ‘Auth_IMAP Connection String’,
‘IMAP_SERVER_EXPLAIN’   => ‘See php.net/imap for syntax, example is: {localhost/pop3:110}INBOX’,
‘IMAP_ALTSERVER’                => ‘Alternative Auth_IMAP Connection String’,
‘IMAP_ALTSERVER_EXPLAIN’        => ‘Same syntax as above, tried if user not valid from first connection string’,
));

To  activate the IMAP Authentication under the phpBB3 Administration Control Panel go to Client Communication - Authentication and fill in the form.
Let me know if you found this helpful.