Add SMTP as common initialism.
Add SMTP as common initialism.

Fixes #49.

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

JekaMas/hint,zimmski/lint,tmc/lint,kovetskiy/lint,darkliquid/lintci,elgris/hint,golang/lint,hellosuper/gohint,NichoZhang/lint,lutzky/lint,gorcz/lint