Unwarranted invocation of starttls

[Eli Zaretskii]

My Emacs was not configured and built (on Windows) with GnuTLS.  Nor
do I need it to send email via my ISP's mail server.  Nevertheless, as
soon as I tried to send email after installing the GnuTLS DLLs and
executables, Emacs said it is invoking STARTTLS to communicate to my
SMTP server.  Of course, this failed.

Why is GnuTLS being invoked, and what should I add to my configuration
files to avoid that and still have GnuTLS installed (for building and
testing)?

TIA