smtpmail-stream-type

smtpmail-stream-type is a variable defined in `smtpmail.el'.
Its value is

nil

Documentation:
Type of SMTP connections to use.
This may be either nil (possibly upgraded to STARTTLS if possible),
or `starttls' (refuse to send if STARTTLS isn't available), or `plain'
(never use STARTTLS), or `ssl' (to use TLS/SSL).

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.