Automated Emails from Shop to external domain emails not working with PS 9.0.2

[Joey]

We upgraded our site from 8.2.3 to 9.0.2 and ever since NO emails are getting through to customers. When we send a test email on the email page under advanced parameters it will only send to internal email addresses... external addresses do not received emails. We kept a backup of 8.2.3 and the emails send fine to external emails from the same page. The configuration is exactly the same.. we tested using SMTP and usr/bin mail and both work on 8.2.3 but not in 9.0.2

We're wracking our brains here... there's no errors and we can't find any logs with any errors. Customers aren't getting order confirmation, shipping confirmation or password reset emails etc.

If anyone has any insight it would be appreciated.