Relevant Products: Exclaimer Cloud - Signatures for Office 365
If you recall a message that has been routed to Exclaimer Cloud, to add a signature, you receive a Recall notice informing you that the message failed to recall successfully (despite the message being unread in the recipient's Outlook mailbox).
Microsoft has confirmed that any messages routed via a connector are likely to fail to be recalled. This is 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 it affects any route or service utilizing connectors - according to Microsoft this is behaviour by design. As a result, recall messages cannot be guaranteed when messages are routed to Exclaimer Cloud.
Unfortunately, there is no workaround at this time and you'll need to raise this issue with Microsoft.