The domain is a brand new domain we've been switching to and we've hit a fun snafu with the DNS configuration recently - looks like some email got mixed up in that. It should be working now, or you can try the same email address at joinzest.com as well.
The response from the remote server was: 554 5.7.1 : Client host rejected: Address locked or deactivated; see http://mail.messagingengine.com/docs/locked.html