message-mail-user-agent

message-mail-user-agent is a variable defined in `message.el'.
Its value is

nil

Documentation:
Like `mail-user-agent'.
Except if it is nil, use Gnus native MUA; if it is t, use
`mail-user-agent'.

You can customize this variable.

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