"smtp; 4.1.0 Unknown adress 450-'4.7.1 Client host rejected cannot find your hostname"
Sounds to me like your server is verifying their mail servers identity by doing a rDNS lookup of the host name and comparing it to the MX records.
The problem is not on your end, but on their improperly configured mail servers usually due to incorrect or missing rDNS or MX records.
http://support.bluetie.com/node/402