Dovecot supports NTLM and GSS-SPNEGO authentication mechanisms using Samba's winbind daemon. It is useful when you need to authenticate users against a Windows domain (either AD or NT).
By default NTLM mechanism is handled internally. You can use winbind instead by setting:
auth_use_winbind = yes
The usernames, returned by winbind, can contain some domain part (either "DOMAIN\user" or "email@example.com"). Such usernames are always transformed to the form of "user@domain". To strip domain part (to obtain corresponding local username, for example), set:
auth_username_format = %n
Dovecot needs path to Samba's ntlm_auth binary to perform the authentication. You can change the path with:
auth_winbind_helper_path = /usr/bin/ntlm_auth
Dovecot currently does blocking lookups, so if ntlm_auth is slow on responding (e.g. network problems), Dovecot blocks all other authentication requests until it's finished.