weird mail bounces

[http://dummy.us.eu.org/robert (Robert )]

I just recently started getting the below error when trying to send mail.

Any ideas how to fix it?  I assume that this means I am unable to send
mail.

 <http://www.yyyy.zzz/~xxx>:
 64.142.7.162 said: 550 5.7.1 <http://www.yyyy.zzz/~xxx>... Relaying denied. Proper authentication required.