Your message did not reach some or all of the intended recipients.

      Subject:    Test
      Sent: 08/08/2011 10:00

The following recipient(s) cannot be reached:

      ‘test@test.com’ on 08/08/2011 10:00
              None of your e-mail accounts could send to this recipient.

This error often occurs when user tries to send an email after copying recipient address from a hyperlink (or clicks on an email hyperlink). Although address may appear normal in Outlook, actual copied address includes mail-to: prefix. This sets Email Type to Mail-To instead of SMTP which Outlook fails to send. On top of that, this invalid address is saved to Outlook Auto Complete list and is resolved every time user tries to send an email to this recipient.

To check Mail Type, double click on recipient email address in TO field and check Email Type (in Outlook 2010, additionally, you will need to click on More Options > Outlook Properties)

To fix this click on Internet Type button to change Email Type to SMTP.
Alternately delete incorrect entry from Auto Complete list (start typing email address, select incorrect entry and press DELETE). Then type full email manually or copy as a plain text from notepad or similar.

Outlook 2003
Outlook 2007
Outlook 2010