M365 Exchange Connector Blocklisting

Question

I am writing an Exchange Connector that will send messages from M365 to my app server, which will then relay modified messages back to M365.

I understand that without Enhanced Filtering for Connections, when the message is returned to M365, the IP address of my appserver would be set as the message source IP. Consequentially, if EOP Anti-Spam checks (which run after the connector/transport rules) mark the message as spam, it would look like my app server is a spammer and potentially get blocklisted.

Installing Enhanced Filter avoid this, but the lack of clarity in the M365 documentation about the order of execution of EOP makes it difficult to spot any other potential pitfalls that could result in my app server being blocklisted.

If anyone has worked on a similar style of connector and encountered any trapdoors like this, please comment thanks...

Answer 1

EOP will filter emails with the following services:

• Connection Filtering
• Anti-Malware
• Mail Flow Rules
• Content Filtering(also known as Anti-phishing policies): Anti-Spam policies & Anti-Phishing policies

Enhanced Filtering for connectors allows IP address and sender information to be preserved, make sure the relay modified messages won't be marked as spam by the configurations(e.g. IP Allow/Block List) of these services in EOP.