I added a 2nd FQDN to an SBS2011 server a few months back. All was well. (Following: http://blogs.technet.com/b/sbs/archive/2011/04/13/how-to-configure-sbs-2011-standard-to-accept-e-mail-for-multiple-authoritative-domains.aspx
I have have the first new user since then, added from the Console, but it fails at the last step of the user setup and the user cannot accept external email.
This message was created automatically by mail delivery software.
A message that you have sent could not be delivered to one or more recipients. This is a permanent error. The following address failed:
<firstname.lastname@example.org>: 550 5.1.1 <email@example.com> Recipient not found. <http://x.co/irbounce
I re-ran the email address policy. But that did not resolve the issue.