pop3_auth plain;

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

plain

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

pop3_capabilities TOP USER UIDL;

Allows to specify the POP3 protocol extensions list to be passed to the client upon issuing the CAPA command. Authentication methods specified in the pop3_auth and (SASL extension) and STLS directives, are automatically added to this list if the starttls directive is enabled.

It makes sense to specify extensions supported by POP3 backends to which clients are proxied (if this extensions are related to commands used after the authentication, when nginx transparently proxies the client connection to the backend).

The current list of standardized extensions is published at the www.iana.org.