Is this error only from certain outlook users to all email destinations you try or only certain destinations? Try this first. Click "Resend This Message", double click the email address, click on the "Internet Type" button and change the type to SMTP. This can happen if your custom address type for an address is switched to mailto instead of smtp (which could be a result of copy-paste the address instead of typing it when you enter it into the address book initially, however I was under the impression this had been fixed in 2010).