I used to get this when my ISP was blacklisted by some Mail Providers. It appeared that someone was sending out large amounts of Spam through the Email Provider and we all got blocked. You could start by contacting the recipient to see if there's something they can do with their Email provider. You could also reboot your PC to obtain a new IP address for your connection. You could also look at your email to see if there is anything in it that might be identified as spam. Such as the use of many recipients under Bcc and Cc fields (I presently get emails rejected if I Bcc too many people).