Okay, Sounds like you have basic IM/Group IM setup and operating normally.. To Review, here is what your should have setup.. Quick Scenario: Company: Contoso Public FQDN: Contoso.com Internal FQDN: contoso.local Internal OCS: OCS01.contoso.local You want users to utilize their e-mail address across the board (user@contoso.com) Internal DNS: A Record for OCS01.contoso.local A Record for sip.contoso.com pointing to IP of OCS01 SRV Record for _sipinternaltls._tcp.contoso.local (Port: 5061 Host OCS01.contoso.local) SRV Record for _sipinternaltls._tcp.contoso.com (port: 5061 Host sip.contoso.com) Your certificate should be configured for the actual Hostname (OCS01.contoso.local) with Subject Alternative Names for sip.contoso.com Send me a privat emessage and we can discuss this in more detail, standard edition should work fairly easily once you get the DNS side up and running.