EV Archive / Search in OWA through Sophos UTM

We recently migrated from TMG Firewall to Sophos UTM and I cannot figure out how to get archive explorer and evault search to work in OWA for external users with the Sophos. There were some very specific guidelines provided for the TMG from Symantec/Veritas, but I cannot find anything useful for the Sophos UTM firewall.

I did see a post (closed) that referenced this issue, but the resolution was unclear to me.

We are on Exch 2010 and Evault 10.3.

Does anyone have this working?

Thanks!