What is it ?
If you are migrating your emails to #Office365 and for any reason if you required by your business to route your email via on-premises network infrastructure, this how you want to do this.
Don’t place any servers, services, or devices between your on-premises Exchange servers and Office 365 that process or modify SMTP traffic. Secure mail flow between your on-premises Exchange organization and Office 365 depends on information contained in messages sent between the organization. Firewalls that allow SMTP traffic on TCP port 25 through without modification are supported. If a server, service, or device processes a message sent between your on-premises Exchange organization and Office 365, this information is removed. If this happens, the message will no longer be considered internal to your organization and will be subject to anti-spam filtering, transport and journal rules, and other policies that may not apply to it.