Relevant Products: Exclaimer Cloud - Signatures for Office 365
You wish to apply signatures to emails sent as shared mailboxes.
Server-side signatures can be applied to emails sent as shared mailboxes in the same way as you would do for a standard user:
Applying a signature to users and/or groups in Exclaimer Cloud - Signatures for Office 365
In order for the data for shared mailboxes to be successfully synchronised with Exclaimer Cloud, a shared mailbox must not be disabled in Active Directory or have Sign-in blocked in Office 365. Microsoft have confirmed that a license will not need to be applied to a shared mailbox providing that the account is not logged into. For more info see: Signatures are not being applied to emails from shared mailboxes