A matching connector cannot be found to route the external recipient

Question

We have an Exchange 2019 hybrid server for remotely enabling email for AD users and sending outbound emails to our mailbox. Although our users are all AD bound, we directly create some mailboxes (such as shared mailboxes) in EOL/M365 (i.e. without AD objects).

The hybrid server reliably sends emails to all AD bound remote mailboxes, but once we change the recipient to a shared mailbox created directly in EOL (without AD objects), we get the following:

<550 4.4.7 QUEUE.Expired; message expired in unreachable destination queue. Reason: A matching connector cannot be found to route the external recipient>

In addition, it is worth mentioning that I noticed in the message tracking log that when we“ user@ourcustomdomain.com ”Successful delivery of the address will cause the routing source to resolve to“ user@ourtenant.mail.onmicrosoft.com ”. Our shared email is created directly in EOL, and by default only“ sharedmailbox@ourcustomdomain.com ”.

When writing this article, I attempted to add through EOL“ sharedmailbox@ourtenant.mail.onmicrosoft.com ”Assist with SMTP and send another test email, but the hybrid host still considers the shared mailbox as an external recipient.

If anyone has experienced this situation and has any suggestions, I would greatly appreciate it. thank you!

Answer 1

Hi, @TomCruise-7376

From your description, I understand that you are experiencing an error when sending an email to a shared mailbox that was created directly in EOL/M365 without an AD object.

After reviewing your error code, there is a problem with the mail routing.

You can troubleshoot in order:

1.Create a send connector to send to the internet. More information can be found Exchange - Error 'A matching connector cannot be found to route the external recipient' | PeteNetLive

2.Make sure that your DNS records (autodiscover, MX records, and so on) are set up correctly and point to the correct environment.

3.Verify that the hybrid configuration and connector are set up correctly to route email to Exchange Online. This includes ensuring that:

-The send connector in Exchange on-premises is correctly configured to route mail to Exchange Online.

-The connector in Exchange Online is set up to accept mail from Exchange on-premises.

4.You can try adding the secondary SMTP address (sharedmailbox@ourtenant.mail.onmicrosoft.com) to your shared mailbox in EOL. You mentioned you tried this, but make sure that the primary SMTP address remains as sharedmailbox@ourcustomdomain.com and that the ourtenant.mail.onmicrosoft.com address is added as an alias.

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