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So much discussion for such a little issue. :-)

A misconfigured server must be part of your scenario.  It's
the only case were the hostname makes any difference.  Using
localhost.localdomain will work find on 99.99% of mail
servers.  For the remaining 0.01%, using socket.getfqdn() has
a higher chance of working than using localhost.localdomain.
If socket.getfqdn() can find a hostname that resolves
back to the IP of the client side of the connection then
it works.  Using localhost.localdomain in that case will
not work.

If socket.getfqdn() cannot find the FQDN (due to NAT,
tunnelling or whatever) things work just as well as if
localhost.localdomain was used a default.  Changing the
default to localhost.localdomain fixes nothing!

getfqdn() is a hack because it's relies on DNS. People
always screw that up. :-)

Regarding your suggested API change, I don't see how it
would help.  I doubt any code actually passes
socket.getfqdn() to SMPT.helo().