Shared mailbox cannot send as

Verify Send As versus Send on Behalf permission and propagation.

How to troubleshoot

1. Check Microsoft 365 service health and capture the exact error and affected account.
2. Verify Send As versus Send on Behalf permission and propagation.
3. Retest with the affected user and document the admin change in your change log.

When to get professional assistance

If these steps do not resolve the issue, or you suspect failing hardware, data loss, a security incident, or a production-system problem, consider qualified technical support. Back up important data where possible and obtain authorization before changing business systems.

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