When recalling a message that has been routed to Exclaimer Cloud to add a signature, the Recall notice informs you that the message failed to recall successfully, despite the message being unread in the recipient's Outlook mailbox.
Microsoft have been able to confirm that any messages routed via a connector are likely to fail to be recalled due to the way that the SearchKey property, which is used to locate the message, is modified during the routing.
This is not unique to Exclaimer Cloud but would affect any route or service utilising connectors and Microsoft suggest that this is behaviour by design. As a result, recall can therefore not be guaranteed when messages are routed to Exclaimer Cloud.
Unfortunately there is no workaround to this issue at this time, we will update this article if more information comes to light.