During a migration, Microsoft Exchange Server 2007 ActiveSync users cannot download messages from mailboxes if proxy requests are made through Microsoft Exchange Server 2013. When this problem occurs, users receive HTTP status error code 503 on Exchange 2007 Client Access servers (CAS).
This problem occurs primarily during peak load hours.
If you use some form of pre-authentication for ActiveSync requests, requests that are proxied on the Exchange 2013 servers are no longer preauthenticated when they are again handled by Exchange 2007.
This behavior causes a significant load increased on the Exchange 2007 servers. This can cause the ASP.Net request queues to fill up.
To resolve this problem, increase the ASP.Net request queue limit in the web.config file on all Exchange 2007 CAS. To do this, run the following code: