Did you ever get this working any better?
Not sure if this applies to EV10 but the below is from the the EV11.0.1 CHF4 readme, is your target domain defined as an FQDN?
Defining an Exchange target domain using the domain flat name caused poor performance for Exchange 2013 archiving [3801638]
In the Administration Console, if you defined an Exchange target domain using only its domain flat name, this caused poor performance for archiving from Exchange 2013 servers. This was most apparent in Exchange journal archiving. For example, if you defined the target domain as exchdom rather than exchdom.local, for connections to an Exchange 2013 server called exchsrv, the Exchange archiving agents tried to use exchsrv.exchdom as the server's FQDN, rather than exchsrv.exchdom.local. This caused delays in name resolution which caused poor archiving performance.
This has been fixed. Enterprise Vault now attempts to determine the correct fully qualified domain name for the Exchange 2013 server during the configuration of MAPI profiles.