keeping getting policy violation email error message when sending a group email

I'm sending a BCC email to a group of greyhound adoption members alerting them to a Meet and Greet.  I have two separate email lists, each containing 54 names, all different.  One note went out fine.  the second note returns the error message - 

Sending of the message failed.
An error occurred while sending mail. The mail server responded:  5ROnxY5ycoNw95ROnxangR policy violation, try again later (DFTD). Please check the message and try again.

Why did the first one go through while the second one was stopped?  It's exactly the same content.  I've send other emails to these two distribution lists within minutes of each other without a problem.  Why can't I do so now?