DerbyDad03 wrote: - show quoted text -
"What's the R factor of that flap vs. the R factor of the rest of the door?
Assuming a quality door, of course. "

If rest of the dwelling is well insulated and
weather stripped, especially roof/attic,
infiltrations issues through the mail slot
will be minimal.

As a test, in my two-story house with attic,
on a cold day I cracked open a down stairs
window. Istuck my hand in the opening and felt cold,
but not coming in. I then went upstairs and
opened the attic stair-hatch about halfway down,
then returned to cracked open window downstairs.
Cold air was blasting in as heated air escaped
into that attic! The moral? Insulate top-down.
Dodges drafts!