Force Send As Public folder to only internal email

I have a mail enabled public folder set up.
I have set up the Send As Permissions accordingly.
Everything works perfectly EXCEPT:

I only want this public folder to send internal emails.

Is there a setting somewhere that will prevent this folder sending to internet email addresses and only send to our domain email addresses?
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imkotteesConnect With a Mentor Senior Messaging EngineerCommented:
Manpreet SIngh KhatraSolutions Architect, Project LeadCommented:
You can use Recipient filtering or even transport rule as Imkottees has said

- Rancy
gpchicago08Author Commented:

Why would my pub folder stop sending emails?  I haven't tried anything yet.

I'm getting this error:  You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk.

It worked fine a while ago.  I sent emails to both internal and external addresses?!?
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imkotteesSenior Messaging EngineerCommented:
make sure that the send-as permission is set correctly.
gpchicago08Author Commented:
Do I need to check the send-as somewhere other than on the public folder?  Because the group I have assigned is still in there.
gpchicago08Author Commented:
So I set the send as permission on the public folder to just myself and it still doesn't work.

This is weird.
gpchicago08Author Commented:
OK.  The auto complete selection is the culprit.  If I use the actual folder every time, it will send.  If I use the email address for the folder in the auto complete, it fails.
I'll start an additional question.

Now back to setting up the Transport Rule...
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