pop3_auth

Sets permitted methods of authentication for POP3 clients. Supported methods are:

plain

USER/PASS, AUTH PLAIN, AUTH LOGIN. It is not possible to disable these methods.

apop

APOP. In order for this method to work, the password must be stored unencrypted.

cram-md5

AUTH CRAM-MD5. In order for this method to work, the password must be stored unencrypted.

external

AUTH EXTERNAL (1.11.6).