smtpmail-warn-about-unknown-extensions

smtpmail-warn-about-unknown-extensions is a variable defined in `smtpmail.el'.
Its value is

nil

Documentation:
If set, print warnings about unknown SMTP extensions.
This is mainly useful for development purposes, to learn about
new SMTP extensions that might be useful to support.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.