Also, as long as you have port 443 pointed to your SBS, then the SonicWall would not be answering on that port, and the certificate would indeed be coming from your SBS. You can see the particulars if you view certificate, and then look on the Details tab > Issuer.
You should have these Containers listed in a properly created certificate:
CN = mail.domainname.com
CN = companyweb
CN = server
CN = localhost
CN = server.domainname.local
(you only put in mail.domainname.com in the wizard, and it creates the other four for you).