We have an Exchange 2019 Hybrid server to remote mail-enable our AD users and send outbound to our mailboxes from certain legacy apps. Though our users are all AD-bound, we create some mailboxes (e.g. shared ones) directly in EOL/M365 (aka no AD object).
The hybrid server reliably sends mail to all of our AD-bound remote mailboxes, but as soon as we change the recipient to a shared mailbox created in EOL directly (no AD object), we get something like this:
<550 4.4.7 QUEUE.Expired; message expired in unreachable destination queue. Reason: A matching connector cannot be found to route the external recipient>
Also, for what it's worth, I noticed in the message tracking logs that when we address to "[email protected]" the successful deliveries do a routing source resolve to "[email protected]". Our shared mailboxes are created in EOL directly and would only have "[email protected]" by default.
As I was writing this up, I tried adding a secondary SMTP of "[email protected]" through EOL, and sending another test message, but the hybrid host still considers the shared mailbox as an external recipient.
If anyone has experienced this and has a suggestion, I'd be grateful. Thanks!
The issue was the send connector. (This host had replaced a 2016 Hybrid host) During the course of hybrid setup, two send connectors were auto-created.
Send connector 1:
Send connector 2 (which we updated to get it working):
When we emailed
[email protected]it was forcing resolution to[email protected]. For shared mailboxes where[email protected]did not have another address, this second send connector was now able to be used to send out.There are other options such as merging the send connectors, but we are leaving these separate for clarity and future understanding.
More details on this and related settings can be found here: https://practical365.com/exchange-2016-smtp-relay-connector/
Hope this helps someone else (or me, again, in a year!)