don't use localhost as the HELO on outgoing SMTP connections

It can be configured using `dns.helo_hostname` or will be set to `dns.smtp_server_hostname`

Related to #19

config/postal.defaults.yml

@@ -68,6 +68,7 @@ dns:
   return_path: rp.postal.example.com
   route_domain: routes.postal.example.com
   track_domain: track.postal.example.com
+  helo_hostname: # By default, this will be the same as the `smtp_server_hostname`
   dkim_identifier: postal
   domain_verify_prefix: postal-verification
   custom_return_path_prefix: psrp