Retain internal exchangemai header

Question

What does this option actually do?
Without it the EOP move most of the mail flow to quarantine.
Thanks

Answer 1

That will retain any Exch headers - essentially it ensures the messages sent within the connector are trusted - usually recommended and explains why if not checked, the messages end up in quarantine

https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/integrate-office-365-with-an-email-add-on-service

Preserves internal headers in messages that are sent to the email add-on service, which means the messages are treated as trusted internal messages. If you select this value, you'll also need to use the same value on this setting for the inbound connector that you create in Step 4 (otherwise, the inbound connector will remove the internal Exchange headers from the returning messages).

Answer 2

The returning message from the 3rd party service.

So imagine you send an authenticated message from a 365 mailbox and it goes through using that 3rd party service outbound connector, then its returned by that service to 365 through an inbound connector. If the internal headers arent preserved, then 365 will not know its an authenticated message originally and it may end up treated as SPAM or quarantined.

The option to retain the internal Excange headers should be enabled for both the outbound and inbound connectors for that service.