How do I suppress mail sending via Thunderbird?

[Skip Montanaro <skip@pobox.com>]

I'm in the midst of migrating from XEmacs to GNU Emacs on my Mac.  My
current stumbling block is sending mail.  I use VM to read mail.  Whenever
I try to send a message from a buffer created by vm-mail it executes
vm-mail-send-and-exit.  For some reason that pops open Thunderbird to send
the message.

Now I suppose this would be fine if I actually liked Thunderbird.  I use
it only for work email, and curse it repeatedly, multiple times per day.
In XEmacs I was using an older version of VM (8.0.2.something).  I
installed the current version (8.1.93a) in GNU Emacs.  I dug into the
source, but couldn't figure out how it passed off the message to T-Bird.
How do I get VM to just send the dang message itself?

Thanks,

Skip Montanaro