here's the issue I did a swing migration from sbs 2003 to sbs 2011. Exchange hosts 3 domains. All three domains were working for several days. I rebooted the server last night. Today I can email, from an external source, only two of the three. All accounts at the third domain get 'no such user' bounce back and yes, internal email is just fine.
ps: fyi I can't get vpn going (error 691) and have been in the network security policy but have, to my best knowledge, set all back to original settings. not sure if that could cause an issue but wanted to communicate it.