In this scenario, we have users who use a primary user mailbox as well as a shared mailbox. They wish to have default signatures for each mailbox. However, when going into Outlook Options > Mail > Signatures, only the user mailbox appears as selectable for configuring a signature. The shared mailbox does not have a way to select a default signature.
Does anyone know how to configure a default signature for a Shared Mailbox?
Any help would be greatly appreciated.
Outlook on the Desktop requires a mailbox to be added explicitly (manually in the instructions below). Not implicitly via Auto Provisioning. Only then are you able to set it a signature as a default.
Instructions on how to do it are here: https://support.microsoft.com/en-gb/office/open-and-use-a-shared-mailbox-in-outlook-d94a8e9e-21f1-4240-808b-de9c9c088afd
Based on my knowledge, you could add a disclaimer as a signature to the shared mailbox by creating a transport rule(Append the HTML disclaimer / Apply a HTML disclaimer to the message). The following rule is for your reference:
If you grant the Send As permission of the shared mailbox to users, after they "send as" the shared mailbox, the recipients will receive emails with a disclaimer(signature):