I have similar problem. I can recieve email, but not send. I get "421 Cannot Connect to SMT".
Everything was working perfectly until I upgraded to Office 2010 and outlook 2010 from outlook 2003. I was on the phone with my Verizon DSL ISP tech support for an hour attempting to fix it, verifying all settings were correct and trying different port numbers. I reloaded Office and Outlook 2010, moved some mail to a new .PST file, nothing works.