Configure email security gateway

Presume it's a hybrid? I'd say you would need the 365 & on prem to be linked so all outbound emails come from the on prem (or vice versa, but not sure thats a supported setup). Then have your inbound & outbound as on prem.

Hello Ruben,

Thank you for contacting the Sophos Community.

I'd recommend you to reach to your Sales Engineer, or Professional Services, for hybrid setups.

As of your questions, 

1. It would be the Public IP of FQDN where Central will be sending the email to, which is usually the Public IP of your router, protecting your exchange server. In the case of being a O365 it would be the server name that O365 provides you with  <yourdomain-com>.mail.protection.outlook.com

2. This is the IPs for outbound email from your organization, where Central expects traffic to come from, usually the Public IP of your router.

Regards,

Thank you for your response Emmanuel,

I will contact our Sales Engineer for hybrid setup, the configurations you describe above works

Have a nice day

Thank you Ian,

Appreaciate it,

Have a nice day

I'd say if both the Exchange and the O365 are the same domains.

Setup the gateway for O365 outboud and use MX on the inbound.