Have you been able to figure out this problem? I'm running into something very similar. We have a domain centrifugeit.com and an internal domain windows.centrifugeit.com. The IIS running SMTP is on windows.centrifugeit.com and can send to all addresses except those on centrifugeit.com.
Any help would be appreciated. Feel free to send a direct e-mail to jhegedus@centrifugeit.com.
Thanks,
Jeff