In a typical scenario one would override the relayhost setting for address verification probes and leave everything else alone:
/etc/postfix/ main.cf: relayhost = $ mydomain address_verify_relayhost = ...
Sites behind a network address translation box might have to use a different SMTP client that sends the correct hostname information:
/etc/postfix/ main.cf: relayhost = $ mydomain address_verify_relayhost = address_verify_default_transport = direct_smtp /etc/postfix/ master.cf: direct_smtp .. .. .. .. .. .. .. .. .. smtp -o smtp_helo_name=nat.box.tld
