Here is what I have done. It seems Outlook 2007 is unable tosend out the Task Request.

1) Created a new task.
2) Clicked on the Assign Task button.
3) Selected one of the contacts from the Task Recipient.
4) Clicked on the Send button.
5) Got the following error message

Your message did not reach some or all of the intendedrecipients.
Subject: Task Request: test
Sent: 1/13/2012 12:55 PM
The following recipient(s) cannot be reached:
John Doe ( 1/13/2012 12:55 PM
None of your e-mail accounts could send to this recipient.