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I'm skeptical about the effectiveness of providing overrides
through defaulted arguments; this is something the author of
the program using smtplib must anticipate and give its user
an option to override. (And because of that, I'm at best -0
on adding the local_hostname argument to the constructor, as
Neil checked in.)

I now agree that leaking the fqdn isn't much of a provacy
breach.

I agree that fqdn w/domain literal fallback is the best
compromise.