Outlook connectivity issues (Exchange 2013, MAPI/HTTP)

Hi,

We're having an issue with some Outlook 2013 clients dropping their connection to Exchange 2013 (both SP1). Connections are made using the MAPI/HTTP protocol, via a reverse proxy. When the connection is dropped, the proxy logs simply show that the connection was reset ("connection reset by peer").

There is no obvious cause for this, or related errors on the Exchange server's event logs. What could be causing this?