I have a SBS 2008 with exchange 2007 not sending email
This is the error I see in the queue
451 4.4.0 primary target ip address responded with 421 4.2.1 unable to connect
After I did a test I see that he and all other users are not receiving any emails either.
The email that the users are trying to send are all in the queue with the error above
I has this same problem a few weeks ago on the server and rebooted the server which solved the problem. Today I have already rebooted twice and no luck.
This is the post from a few weeks ago