Outlook Emails Not working, what could it be ?

One of our customer just sent an email asking, why outlook email configuration is not working.
He said, it works with other ISP, but not with ours.

He sent me a screenshot as well.

Is it something Microtik is blocking ?
How to test whats blocked ?

Maybe the smtp server is not accessible from your network…

There was a firewall rule. Removed it and it fixed.