The key to this error is in the message "550 Administrative prohibition". This means the mail server is not accepting this for some reason or another. Check with your Host about this.
I got this very message just a few hours ago. Because of death in the family, I've not sent out mails to my readers in a very long time and therefore haven't checked all the configurations... but during that time I made a host move. To get rid of a bunch of spam, I also retired several email addresses.
WELL. I went back to the list settings. And voila, one of the addies I retired was my Editor @ waybilltoadventure.com, as it was so old and SO on the spammers list. I updated the two places asking for email addresses: Send as Email, and Email address(es) to notify, separated by commas -- and it fixed it immediately.
Obviously, before the message you and I both got, the software checks to make sure these are working addies. Dunno if this will help in your situation but it fixed my problem right off.
