MacOS X postfix relyhost mail setup

main.cf

...

# append to /etc/postfix/main.cf
relayhost=smtp.sendgrid.net:587
smtp_sasl_auth_enable=yes
smtp_sasl_password_maps=hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options=
smtp_use_tls=yes
smtp_tls_security_level=encrypt
tls_random_source=dev:/dev/urandom

org.postfix.custom.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>org.postfix.custom</string>
    <key>Program</key>
    <string>/usr/libexec/postfix/master</string>
    <key>QueueDirectories</key>
    <array>
        <string>/var/spool/postfix/maildrop</string>
    </array>
    <key>AbandonProcessGroup</key>
    <true/>
    <key>OnDemand</key>
    <true/>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <true/>
</dict>
</plist>

sasl_passwd

smtp.sendgrid.net:587 [email]:[password]

terminal.txt

# create empty file
sudo touch /etc/postfix/sasl_passwd

# set permissions before entering credentials
sudo chmod go-rwx /etc/postfix/sasl_passwd

# setup credentials
sudi vi /etc/postfix/sasl_passwd

# create lookup table
sudo postmap /etc/postfix/sasl_passwd

# for extra security, remove file once lookup table is created
sudo rm /etc/postfix/sasl_passwd

# create a backup copy
sudo cp /etc/postfix/main.cf /etc/postfix/main.cf~orig

# append config
sudo vi /etc/postfix/main.cf

# create service config
sudo vi /Library/LaunchDaemons/org.postfix.custom.plist

# add mac os service service (be sure correct permissions are set; root:wheel)
sudo launchctl load /Library/LaunchDaemons/org.postfix.custom.plist

# restart macOS to confirm auto loading on boot

# confirm postfix process is running
ps -A | grep postfix

tested macOS High Sierra 10.13.5