git send-email says AUTH is unknown command?

[Phillip Susi <phill@xxxxxxxxxxxx>]

git send-email prompts for my SMTP password then says:
Command unknown: 'AUTH' at /usr/lib/git-core/git-send-email line 1573.

That line reads:

smtp_auth_maybe or die $smtp->message;

Why is it complaining that it does not know what AUTH means?  It is
connecting to port 25 and /should/ be using STARTTLS.