Not sending registration email

O/S: Ubuntu 18.04 Server
MTA: postfix (Zimbra)
Zimbra ver: 8.8.15

Mastodon instance installed but not sending registration information: is this setup right so far. I know you can’t vouch for the Zimbra setup

SMTP_SERVER=smtpout.topic-master.com
SMTP_PORT=587
SMTP_LOGIN=xxxxxx@justourmail.com
SMTP_PASSWORD=XXXXXXX
SMTP_AUTH_METHOD=plain
SMTP_OPENSSL_VERIFY_MODE=none
SMTP_FROM_ADDRESS=xxxxx@topic-master.com
SMTP_ENABLE_STARTTLS_AUTO=true
SMTP_TLS=true