Error updating public folder busy exchange 2016
Another option is to set the location from which the Office 365 is hosted for the company. At this point, you need to enter credentials of your on-premises admin and its cloud counterpart.
It gives the possibility of central management of mail flow rules and signatures throughout the company.
Here, the Office 365 Hybrid Configuration Wizard lists your domains along with information if the Autodiscover service is available.
From the domains’ list, you have to choose your public domain or domains, remembering that Autodiscover has to be configured correctly for them.
The server should have appropriate SMTP certificate on port 25.
This port also cannot be blocked by any firewall software or by the router.