I guess, below answers the last question.

Maybe good to consider adding this to: http://nginx.org/en/docs/mail/ngx_mail_core_module.html



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).