Sometimes when you try to send an e-mail from within Outlook you may get a pop-up window telling you: Cannot Send This Item
This is usually due to either a malformed email address that the server cannot understand or it is due to an email signature block somewhere in the email that has also somehow been malformed The solution is to change the format of your message to rich text.
In Outlook 2007:
In Outlook 2010:
You should now be able to send the message without any problem.