Its a known issue that mail flow certain domains does not work if your exchange server 2007 satisfies the following condition
* Os : windows 2008
* Exchange : Exchnage 2007
I tried the following steps and end up finding few things
1. Mail flow will work to all the domains except few
2. When you smart host to that particular domain, the mail will go through
3. When you do a telnet and drop a mail to the remote domain, mail will go through
This can be resolved by running the hot fix
Mail flow to certain domains does not work when you run Exchange Server 2007 on a Windows Server 2008-based computer
When a mail is generated to a remote domain (problamatic domain), the server will query for the AAAA record.
Which is odd.
You can test this by running a netmon trace.
So it is obvious that mail will not go through because the remote server will not responsd to the AAAA record query
At times running the hot fix won’t fix the problem. So, Just create a new send connector and add the remote server ip as a smart host for time being
This also happened to me for Exchange server 2010 as well.