Exchange Server 2013 Hybrid Configuration Send Connector Issue

Gianpaolo Panucci 1 Reputation point
2022-03-30T11:47:58.27+00:00

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:

  1. MX is pointed to Mimecast. Mimecast is pointed to On-Prem Exchange 2013 box
  2. Server is On-Prem with Hybrid Configuration Wizard Installed & Configured
  3. All Prod Mailboxes are On-Prem
  4. We have Migrated a Test Mailbox to Cloud to verify mail flow
  5. We have created Cloud Mailboxes Directly to verify sideways mailflow

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?

Exchange Server Management
Exchange Server Management
Exchange Server: A family of Microsoft client/server messaging and collaboration software.Management: The act or process of organizing, handling, directing or controlling something.
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  1. Andy David - MVP 143.8K Reputation points MVP
    2022-03-30T11:54:04.433+00:00

    "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

    https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-to-route-mail

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