We have a load-balanced Exchange 2013 SP1 cluster, running MAPI over HTTP.
Client connectivity inside our own network works just fine, while clients connected over Direct Access does not connect. The Outlook logs on the client show absolutely no error at all.
The Direct Access server is running 2012 R2, the clients are all Windows 8.1. Everything is patched.
I've been searching like crazy the last couple of weeks, and the only interesting hits I get are about TMG 2010 (UAG) filtering out the requests due to the source IP changing (the exchange load balancer). There is a Knowledge Base Article (982604) that describes this, and a rather hefty blog post about the issue from premier support, but sadly the script does not work on our server since it's not TMG and it's Windows Server 2012 R2..
I'm at a loss here. I'll give this question a week, then I'll raise a premier support case with Microsoft.