Resolution was found in KB note from Exclaimer. Specifically the note to select the “Stop processing more rules” option to force the Exclaimer rule to run and to prevent Office 365 from running the *VENDOR* rule until the email is returned from Exclaimer Cloud with a signature attached. At this point, the Exclaimer Transport Rule will have its exception triggered by the Exclaimer message header once it returns, and the *VENDOR rule will be used instead.