Outlook 2007: Unable to send emails

I am using Windows Xp pro with sp2; I recently upgraded office 2003 to office 2007 everything is fine except the outlook module. I am able to receive mails but unable to send any i.e. whenever I am trying to send any mails its giving me an error saying
?The message interface has returned an unknown error. If the problem persists, restart Outlook?
Even after restart the problem is the same, I have tried reinstalling office but the problem remains. I checked my SMTP setting and they are fine. So I tried Google but found nothing of use. Even I tried creating a new profile using
?outlook.exe /promptimportprf \\localfolder\outlook.prf?
But again it?s of no use.
Anyone plz help.