You're not the only person who had this problem these few last days.
The problem comes from Microsoft immediatly, from their servers.
To fix the problem follow this procedure:
1) In the error file that you received:
(example):
It gives you the names from the people who didn't receive your email.
2) Press "New mail"
3)In the "To:" bar, typ the names that appear in the error file (Peter Dost and Raymond Lucassen, here for example)
4) When the entire mail address from the person appears, press the X to delete it.
(Do exactly the same with Raymond Lucassen).
5) Close the mail that you opened to delete the email addresses.
Now, you should be able to send mails to these persons again.
Best regards,