Error message #550 Permanent Failure

Lately I have been receiving this #550 error message.  Can someone tell me why I might be getting them?  The e-mail I send never gets through to the recipient when I get this message.

Lately I have been receiving this #550 error message.  Can someone tell me why I might be getting them?  The e-mail I send never gets through to the recipient when I get this message.