I have given below details in other forum thread as well, it will be helpful for you for future in case of similar problems.
Required configuration of EV for External OWA & “RPC over http/https”
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For Exchange 2010/2007, if we need Enterprise vault functionality from external by OWA or outlook Anywhere (RPC over http/https), Enterprise vault host record must be published.
Details are present in "Symantec Enterprise Vault Outlook Web Access (OWA 2010 and earlier) Internal and External WebApp URLs" http://www.symantec.com/docs/TECH63250
If you have setup firewall other than ISA /TMG (Cisco Pix, Lynxes, watchguard,..etc). then you should have separate published host record for EV server.
Example:-
https://web.domain.com/owa (External IP 200.100.1.1 ) https://CAS.domain.local/owa (internal IP 10.0.0.2)
https://EV.domain.com/enterprisevault (External IP 200.100.1.1 ) http://EV.domain.local/enterprisevault (internal IP 10.0.0.3)
If ISA is in place for link translation, we can use existing published host record of exchange, that can work for EV as well, Example:-
https://host.domain.com/owa (External IP 200.100.1.1 ) https://CAS.domain.local/owa (internal IP 10.0.0.2)
https://host.domain.com/enterprisevault (external IP 200.100.1.1) http://EV.domain.local/enterprisevault (internal IP 10.0.0.3)
EV desktop policy settings for "RPC over http / https Outlook mode".
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Once you configure EV host record publishing, then the changes you need to make on EV server.
Open VAC \ Expand policy \ Exchange \ select appropriate desktop policy \ Right click select ‘properties’ \ Advance \ next to ‘List setting from’ select ‘outlook’ \ make change below settings.
RPC over http restriction. = None
RPC over http connection = use proxy
RPC over http proxy URL = https://EV.domain.com/enterprisevault
Where "https://EV.domain.com/enterprisevault" is External EV published Name
Synchronize all user in EV server.