How to deny user send to external email domain except one email domain

My client want to only allow theiremail domain send to (Partner company) user cannot email email to any other external address except

Transport Rule is set as below

Form a member of Distribution list
and send Delivery not authorized, message refused to sender with 5.7.1
except when the text "to" appears in a

Test results
1. User send an email to -- denied
2. User send an email to -- allowed
3. User send an email to and to - email sent out.

How to deny test result 3?
Who is Participating?
MesthaConnect With a Mentor Commented:
I haven't seen a way to do this. It is one thing where Exchange 2003 SMTP connectors worked well.
The only option would be to put all email addresses in the other domain in to your domain as Contacts, then add them to a group and restrict to that.

It is either specific email addresses or text match. As your text appears, it is passed.

MunndayCommented: have a read over that artical - it should solve your problems
YalanAuthor Commented:
Thanks, I read that article before it's talking more about Exchange 2003
In my case is using Exchange 2007
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Alan HardistyCo-OwnerCommented:
There is a similar question in EE that still awaits an answer and as far as I am aware, this cannot be done unless there is a 3rd party app which can do this. 
Consider updating to the latest service pack and installing the latest updates.
"Consider updating to the latest service pack and installing the latest updates."

How is that going to help?

Nothing has changed in the latest updates - I checked on my own system which is fully patched.

Alan HardistyCo-OwnerCommented:
I couldn't be bothered to respond to that comment!
YalanAuthor Commented:
Thanks Mestha, I will try your solution.
Looks like by design we cannot do AND condition to restrict mail flow.
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