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I have recently had an Office 365 and Exchange 2007 Mail Flow Issue for Sub Domain. We have our primary users on [email protected] and have setup Office 365 to process mail for [email protected] .

The setup for Office 365 went extremely smoothly (with a lot of help from a friend – you know who you are J.P.) and I have integrated Office 365 with our on-premise Active Directory using ADFS and Synchronization Service Manager for Single Sign On.

However when a user,  [email protected] tried to send an email to [email protected] using the on-premise Exchange 2007 the mail was not delivered.

User [email protected] could send email to [email protected] without any problems, but again a reply from [email protected] was not delivered.

I investigated further on the Exchange 2007 Server and found that in Queue viewer there was an Unreachable Domain queue with emails which could not be delivered. Continue Reading