Try disabling antispyware/antivirus or firewall installed in your system. Then try forwarding your mails.
You would also require checking your outgoing mail server settings.
profile corruptions can also be a reason for the problem. In such case
you need to repair the outlook profile or create a new one.
If it doesn’t resolve the problem then, try running outlook in safe mode by executing ‘outlook.exe /safe’ command.
For more help go to the page: http://support.microsoft.com/kb/813514