451 4.3 5 server configuration problem - Please urgent !

Hi All,

I'm a new of Symantec Messaging Gateway.

There is on my appliance, an email pending. It is not sent to destination, reporting this error:

All other email are fine, no other errors. So I suppose that the problem in on the destination side.

I'm right ?? There is something that I can do from my side ?

Thanks in advance, cheers.

GianClaudio