Mailbox-bound incoming webhooks stopped working

Question

Connectors can be created not just for O365 groups and Teams channels, but also for mailboxes. With an incoming webhook associated with a mailbox, for example, you can send MessageCard-formatted messages from scripts and LOB applications by posting unauthenticated to an HTTP endpoint, without using SMTP.

This worked until recently. Now posting to a mailbox-bound incoming webhook will appear to succeed, without delivering any message to the user’s mailbox.

To reproduce the issue, create a mailbox-bound incoming webhook connector, and post a MessageCard object to its URL.

Invoke-WebRequest <webhookUrl> -Method POST -ContentType 'application/json' -Body '{"Summary": "Test"}'

You’ll get a 200 response with content “1”—as expected—without actually seeing a new message in your inbox.

Further information:

• There are no log entries for these requests in the Exchange Online message trace.
• Newly created tenants seem to be affected as well.

I believe this change was caused by a recent service-side update which is either unintended or, to my knowledge, undocumented. If I’m missing something please let me know.