This bounce message indicates that the recipient mail server rejected your IP address for unknown reasons. Are your other domains sending from their own IP Address ?
The easiest way to determine why this is failing is to contact the administrator of the domain you are sending to as they should be able to tell you why the connection was rejected. The issue here resides outside of SmarterMail, but with the recipient servers mail policy, and\or a potentially misconfigured domain in terms of DNS records.
You'll want to ensure that the IP you are sending from has valid rDNS records in place, and the sending domain has valid SPF records authorizing your server to send mail for this domain, these can be a common reasons for rejection as well.
Technical Support Supervisor