Hi @William Lay
Welcome to the Microsoft Q&A forum!
Thank you for your detailed description. Based on your explanation, it seems the issue occurs when a user attempts to schedule an email to be sent from a shared mailbox using the "Send As" permission.
From my testing, I’ve noticed that when you first assign the 'Send As' permission in the Exchange Admin Center (EAC), it can take some time, possibly more than 5 minutes, for the change to fully take effect. If you attempt to schedule an email immediately after setting the permission, you might encounter issues. However, after waiting a few minutes, the functionality typically works as expected. As a recommended action, you can try removing and re-adding the "Send As" permission. After doing so, please wait patiently for at least 5 minutes to allow the change to take effect. Then, send a test email to check if the issue still occurs.
In the meantime, please ensure that the shared mailbox is visible in the address list. If you assign the "Send As" permission to a mailbox that’s hidden from address lists, the delegate won’t be able to send messages from it.
For more details and context on managing delegation permissions, you can refer to the following Microsoft article: Manage permissions for recipients | Microsoft Learn
If I've misunderstood any part of your issue, or if you have any updates to share, please don’t hesitate to reply. Your feedback is crucial in helping me find the best solution for you.
Thank you for your understanding
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