Email question...

I'm having a weird issue with mail on my mac. I have an address that I've sent email to numerous times. I sent an email to this address yesterday and received a kicked back email in my junk folder. The email is titled "Mail Delivery Subsystem". It mentions that the email address had permanent fatal errors, the email address, then reason: 579 message content is not acceptable here? I tried again today, same thing. I even tried using aol in safari and when I send it from aol, a few minutes later I get the kicked back email in my junk folder.

Anyone familiar with this?