Module ngx_mail_smtp_module

Directives
     smtp_auth
     smtp_capabilities

Directives

Syntax: smtp_auth method ...;
Default:
smtp_auth login plain;
Context: mail, server

Sets permitted methods of SASL authentication for SMTP clients. Supported methods are:

login
AUTH LOGIN
plain
AUTH PLAIN
cram-md5
AUTH CRAM-MD5. In order for this method to work, the password must be stored unencrypted.
external
AUTH EXTERNAL (1.11.6).
none
Authentication is not required.

Syntax: smtp_capabilities extension ...;
Default:
Context: mail, server

Sets the SMTP protocol extensions list that is passed to the client in response to the EHLO command. 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.