Mail sent to 2 of 3 users bounced to sender. Not sure why, bounce from MAILER-DAEMON@gproxy10-pub.mail.unifiedlayer.com

Mail sent to 2 of 3 users bounced to sender. Not sure why, bounce from MAILER-DAEMON@gproxy10-pub.mail.unifiedlayer.com

Any idea what's going on here?
Thanks.
Here's the bounce that the sender got:


From:  MAILER-DAEMON@gproxy10-pub.mail.unifiedlayer.com  
[mailto: MAILER-DAEMON@gproxy10-pub.mail.unifiedlayer.com ]  
Sent: Thursday, December 08, 2016 5:00 PM 
To: xxxxx
Subject: failure notice 

Hi. This is the qmail-send program at gproxy10-pub.mail.unifiedlayer.com. 
I'm afraid I wasn't able to deliver your message to the following addresses. 
This is a permanent error; I've given up. Sorry it didn't work out. 

<xxxxxx >: 
204.141.32.121 failed after I sent the message. 
Remote host said: 554 5.2.3 MailPolicy violation Error delivering to 
mailboxes