compose-mail-user-agent-warnings is a variable defined in `simple.el'.
Its value is

If non-nil, `compose-mail' warns about changes in `mail-user-agent'.
If the value of `mail-user-agent' is the default, and the user
appears to have customizations applying to the old default,
`compose-mail' issues a warning.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.