Opening a shared mailbox proves that you can access its contents. It does not prove that Microsoft 365 permits you to send from that address. If incoming mail works but a reply fails, check the mailbox’s send permissions before rebuilding the Outlook profile.

Write down the shared address, your own work account and the exact error. Also decide what the customer should see: the shared business identity alone, or your name acting on behalf of it. That choice determines which permission the administrator should check.

Match the permission to the job

Full Access lets a user open and manage the mailbox, including changing and deleting messages. It does not independently allow sending. Send As lets a permitted user send using the shared mailbox identity. Send on Behalf shows that the individual is acting for the mailbox.

Ask an authorised administrator to inspect the assigned permissions for the actual account, rather than repeatedly adding the mailbox in Outlook. If the mailbox is hidden from the address list, mention that too: Microsoft documents desktop Outlook limitations for send permissions in that configuration.

The shared mailbox’s associated account is not intended for direct sign-in. Keep its sign-in blocked and use each staff member’s own authorised account. Sharing a password would obscure the permission problem rather than solve the intended access model.

Verify the result with a reviewed test

After an authorised correction, allow for the change to take effect and prepare a harmless test reply. Inspect the From field before sending it through the normal approval process. Ask the test recipient to confirm the displayed identity.

Check the reply’s arrival separately from its appearance in a local Sent Items folder. Record which work account and client were tested, the observed sender and any remaining error.

If permissions are correct and sending still fails, preserve that evidence for the administrator. It narrows the next investigation without treating every shared-mailbox problem as a broken computer.

References: Microsoft shared mailbox permissions.