Comment 5 for bug 50689

[I'm not sure if this got out before I went on holiday.]

Sorry, I realize the current smtpmail doesn't support auth at all --
one of the many problems with a new Emacs not getting released, but
once you start addressing them all, there's no end to it.

The safest place to get a compatible smtpmail.el will be from the
current Gnus distribution at <URL:http://www.gnus.org/>, since that
can't depend on any Emacs 22 features.