For local mail, it seems to use the 127.0.0.1 line in /etc/hosts and on mine, I have it *analogous to
127.0.0.1 btdig.com localhost.localdomain localhost
And I can type a mail message to root
Subject: Hi root
Bla bla bla
and then read it with
Of course, that is just local mail.
By *analogous, I mean instead of btdig.com, on my system it is my hostname.
I don't know whether this would impact mail other than local mail or not.
And just because it works for me, it doesn't mean that it is correct.