smtp_auth login plain;
Sets permitted methods of SASL authentication for SMTP clients. Supported methods are:
- AUTH LOGIN
- AUTH PLAIN
- AUTH CRAM-MD5. In order for this method to work, the password must be stored unencrypted.
- Authentication is not required.
Sets the SMTP protocol extensions list
that is passed to the client in response to the
Authentication methods specified in the smtp_auth directive
are automatically added to this list.
It makes sense to specify the extensions supported by the MTA 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.