client access issues with migrating exchange 2007 to new hardware

Dear experts,

I am in the process of migrating an exchange 2007 server to new hardware. The source server is a single box with all roles and the new server will be performing the same functions after migration is completed.
So far, I have:
- updated the source server to exchange 2007 SP3
- built the new server, joined to domain and installed exchange server 2007 SP3
- enabled outlook anywhere
- exported the cert from the old server and imported it into the new
- enabled the cert with IIS, SMTP, POP and IMAP

All other settings are as it was out-of the-box.

I have moved a mailbox over to the new exchange server and when outlook opens, it detects the move and repoints the outlook account to the new server.  I also setup outlook on a remote site using rpc over http with a mailbox that was still on the old server - no problem, works fine.  I then closed outlook and moved the mailbox to the new server - again, outlook repoints and all is fine.

So at my firewall, I change the port forwarding for 25, 443 and 80 to point to the new server. When I do this all Outlook LAN stations get a cert error message and remote users with outlook setup using rpc over http, jump to disconnected.  Changing the ports back resumes services to normal.

I'm missing something here that I hope you can help me with.

Thanks in advance.