I believe the LinkedIn account IP is in one location, and when the cookie is used by your servers on Sendspark, it is done from a different location (and IP address). I think the only stable fix will be if you could provide your service with the ability to use a Proxy (IP) for each client. This way, when your server uses the cookie to visit LinkedIn, it uses the LinkedIn account from the same specified proxy. This would ensure a stable connection and prevent LinkedIn from blocking the integration.