Exchange 2010 Certificate Issue

I have to Exchange 2010 2 MBX and 2 HUB/CAS. I hve also 2 Certificate one for External for third party and one Internal for internal users.

I used External Certificate with IIS service assign for OWA , ActiveSync and Outlookanywhere. And internal certificate with SMTP service assign for internal users. I notes now internal users getting certificate error when they open outlook and this error disapper when i assign IIS service to internal certificate but i cant keep IIS service with Internal certificate while i need it for External users. So wht is the solution and why SMTP service not working for internal users.

I tired many time to remove both certificate and import it again with same result.  

Technical Architect