You configure a legacy namespace, but you don't configure your EWS application to utilize Autodiscover.
In this scenario, you intermittently receive "500" error responses from the Exchange Server 2013 mailbox role for EWS traffic that is directed toward mailboxes that are hosted on Exchange Server 2007. These responses can be identified in the Internet Information Services (IIS) log and HttpProxy log of the Exchange Server 2013 Client Access server (CAS) role.
To resolve or avoid this problem, you must use a legacy namespace. We strongly recommended that you use Autodiscover. Otherwise, the decision to use the Exchange Server 2013 namespace or the Exchange Server 2007 legacy namespace based on the location of the mailbox would involve a manual effort that will add to administrative overhead.
In an Exchange Server 2013 and Exchange Server 2007 coexistence configuration, legacy namespaces are required for both EWS and OWA services for client connectivity. The proxying of EWS traffic from the Exchange 2013 mailbox server to the Exchange 2007 CAS is unsupported and will cause inconsistent results.
OWA uses silent redirection to the legacy namespace. However, EWS does not support silent redirection. Therefore, the default behavior for Exchange Server 2013 is to proxy the traffic that is directed to legacy servers. While this works as expected for Exchange Server 2010 EWS traffic, it is not reliable in Exchange Server 2007. Instead, you should manually redirect the EWS client traffic to the legacy namespace.
For more information about client connectivity in an Exchange Server 2013 coexistence environment, see the following Exchange Team Blog article: