==== GMail as a Relay on Linux CENTOS7 ====

You can remove Postfix and use SSMTP as it has easier configuration.

  yum install ssmtp ca-certificates mutt

Follow instructions at https://golb.hplar.ch/2018/06/send-only-email.html 
  * See {{ ::sending_emails_from_your_linux_server.pdf|Sending_Emails_From_Your_Linux_Server.pdf}}

To get desired sender (username@hostname) when using mutt to send, add this line to /etc/ssmtp/ssmtp.conf:
  FromLineOverride=YES

Add these lines to the global /etc/Muttrc.local (easier than adding it to each user's $HOME/.muttrc):

  set from = "`whoami`@`hostname`"
  set realname = "`whoami`@`hostname`"

Successful test via manual send:
  mutt -s "Test4" user@gmail.com </dev/null

Successful test via crontab:
  \* * * * * /path/to/bin/mutt_test.bash > /path/to/logs/mutt_test.log

The mutt_test.bash test script has this for content:

  #!/bin/bash
  date=`date`
  mutt -s "$date bash mutt test" user@gmail.com </dev/null