pop3_capabilities

Sets the POP3 protocol extensions list that is passed to the client in response to the CAPA command. The 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 the extensions supported by the POP3 backends to which the clients are proxied (if these 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 www.iana.org.