Mail Gets Stuck In Delivery Queue 451 4.4.2 [internal] connection closed by remote host

Hi

I have the this problem mostly (99%) with mails sent to Yahoo (451 4.4.2 [internal] connection closed by remote host).I have searched the error code but i could only find problems related to cisco firewalls at the gateway and outbound STMP filtering on the firewall which in our situation is not the case.The SMG is an appliance 8340 with 10.0.1-2 version installed.