It's actually more complicated than that (now that I've thought it over a bit more), since non-FQDN myhostname values are supported.
If I have:
mydomain = example.com
and either:
myhostname = mail.example.com
myhostname = mail.
Then the expanded value of myhostname is mail.example.com either way. Per the Postfix documentation, a non-FQDN my hostname gets mydomain appended to it.
It's actually more complicated than that (now that I've thought it over a bit more), since non-FQDN myhostname values are supported.
If I have:
mydomain = example.com
and either:
myhostname = mail.example.com
myhostname = mail.
Then the expanded value of myhostname is mail.example.com either way. Per the Postfix documentation, a non-FQDN my hostname gets mydomain appended to it.
Just stripping trailing dots breaks that.