Where is the postfix log on Sierra?
This works for me:
log stream --predicate  '(process == &quot;smtpd&quot;) || (process == &quot;smtp&quot;)' --info


Explanation:
--predicate is used to filter the log. In this case it looks for the process &quot;smtpd&quot; or &quot;smtp&quot;
--info shows all messages down to info level, which is good for most purposes.
Hope this helps!