Postfix in combinaton with MAMP
I'm in pretty much the same boat as you. The exception being I'm using mamp 4.5.

I'm yet to find a solution however I can help you to see the mail log.

in terminal you can use:
  log stream --predicate '(process == "smtpd") || (process == "smtp") || (process == "master")' -info

Using smarthost with the default mamp /etc/postfix/main.cf I keep getting:
  master: warning: process /usr/libexec/postfix/qmgr pid 9687 exit status 1
  master: master exit time has arrived

I also get the same message if I turn mamp postfix off and activate it via terminal. For this I have followed https://benjaminrojas.net/configuring-postfix-to-send-mail-from-mac-os-x-mountain-lion/ 

with postconf -n I get the message
  postconf: warning: /etc/postfix/main.cf: unused parameter: mydomain_fallback=localhost

I hope this helps in some way.