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Hi All,
I've got an issue with a clients Exchange 2013 Hybrid Configuration.
Hybrid is brand new, we've finally got approval to go to O365.
Basics:
The issue we have is that when an email is sent to a user account that is located in the cloud environment, Mimecast receives the message, Mimecast then sends the message to the on-prem server, the on-prem server sees that the destination is external and routes via a "*" send connector back to Mimecast which repeats the process and throws the message into a loop; it does not consider the Outbound to Office365 send connector. This is resulting in a message loop.
Error is: "5.4.12 SMTP; Hop count exceeded - possible mail loop detected on message id"
Both connectors have a cost of 1.
I read somewhere that it could be to do with the alphabetical order of Connectors with the same cost, so I changed the name to "Z - Mimecast" - still nothing.
There's no simple priority system that says if it is part of this scope send with this 1st otherwise move to next send connector.
Once we've got all of our mailboxes in the cloud we'll just point directly to Office365, however the company doesn't wish to take this approach incase the migration time takes significantly longer than a weekend.
Office365 support haven't been terribly helpful.
What's the best way to get mail going from our on-prem to our Office365 tenancy?
I guess this phenomenon caused by Mimecast cannot distinguish which mailboxes are hosted on Exchange online.
When emails are sent out from Exchange on-premises, they should use the "contoso.mail.onmicrosoft.com" as routing email address which will not be sent back to Exchange on-premises.
Have a check about your send connector, make sure it is sent email to Internet directly rather than use the Mimecast as smart host.
I am writing here to confirm with you any update about this thread now. Does those information help?
"We have Migrated a Test Mailbox to Cloud to verify mail flow"
Does the test mailbox have a remote mailbox on-prem? If so , then you should have a send connector on-prem for the
contoso.mail.onmicrosoft.com address space and the message would then get routed to the 365 mailbox.
I would also recommend pointing mimecast at 365 rather than on-prem and routing outbound mail through 365 as well.
Create the connectors for in 365. MimeCast prob has instructions specific to their requirements