https://wiki.zimbra.com/wiki/Outgoing_SMTP_Authentication
https://wavemotiondigital.com/2020/03/30/zimbra-smtp-relay-for-multiple-domains-via-mailgun-or-other-provider/
https://www.smtp2go.com/setupguide/zimbra-mta-8-7/

First you'll need a relay service/smart host with a user name and a password to send through.

zmprov ms `zmhostname` zimbraMtaRelayHost mailrelay.example.com

Reboot the ASSP server host, seems to be needed or I'm impatient.

echo mailrelay.example.com username:password > /opt/zimbra/conf/relay_password

Create a postfix lookup table:

postmap /opt/zimbra/conf/relay_password

Test that the lookup table is correct, the following should return username:password:

postmap -q mailrelay.example.com /opt/zimbra/conf/relay_password

Configure postfix to use the new password map:

zmprov ms `zmhostname` zimbraMtaSmtpSaslPasswordMaps lmdb:/opt/zimbra/conf/relay_password

Configure postfix to use SSL authentication:

zmprov ms `zmhostname` zimbraMtaSmtpSaslAuthEnable yes

Configure postfix to use the outgoing servername rather than the canonical server name:

zmprov ms `zmhostname` zimbraMtaSmtpCnameOverridesServername no

Enable Plain Text Authentication (we will use TLS to protect it)

zmprov ms `zmhostname` zimbraMtaSmtpSaslSecurityOptions noanonymous

Enable TLS:

zmprov ms `zmhostname` zimbraMtaSmtpTlsSecurityLevel may