https://apple.stackexchange.com/questions/12387/how-to-send-an-email-from-command-line

1. Open /etc/postfix/main.cf and add the following lines:

    relayhost = [smtp.gmail.com]:587
    smtp_sasl_auth_enable = yes
    smtp_sasl_password_maps = hash:/etc/postfix/sasl_password
    smtp_sasl_security_options = noanonymous
    smtp_sasl_mechanism_filter = plain
    smtp_use_tls = yes

2. Create /etc/postfix/sasl_password

    [smtp.gmail.com]:587 <email>:<password>

3. Change permissions on password file

    sudo chmod 600 /etc/postfix/sasl_password

4. Tail the postfix logs.

    log stream --predicate  '(process == "smtpd") || (process == "smtp")' --info

5. Enable "less-secure" apps

    First, send a test message.  This message will fail if you haven't given permission for "less
    secure" apps.  In that case, you'll both get an error message in the log, and you'll get an
    email from Google. Follow the link in the email and enable the feature.

    If the test message goes through, then your Gmail account was already configured to allow it.

    WARNING: This will slightly weaken the security of that account, so it's best not to do this
    with an account which can receive sensitive information.