accept mails only from extern

You can't really create an email address that will work for outside email but reject email from internal users, since they can also send internet email.

If the goal is to make it so that internal users can't accidentally send messages to this address, you could set the visibility of the account to None. That will prevent the account from showing up in lists of accounts or in the address book. Someone will still be able to send mail to the account by typing in the full address if they know it, but that's going to be true for any internet address.

The only way to prevent messages from specific people from coming into an internet email address is by using some sort of filtering or blocking, either at the server or at the client.

My suggestion was also to make it not visible.   You could also use a rule to respond or delete the internal mail, we are on GW 6.5 so I am not sure if it has gotten easier with 7 or not.
For our resources (rooms, cars) we setup a rule that rejects anything sent to the resource that is not an appointment.  The problem will be limiting the rule to only internal email.  In my vacation rule I have a condition "From [!] @" so that I send replies only  internally.  I am not sure if the same would work for you.  I.e., if mail does not have @ then delete the mail.  
If the mail to this account is from a web form it would be easier because you could also use subject or content in your rule.
If you do use a rule to block the mail, you may want to move it to another directory as the action before making the action "delete and empty" so you can review the action it takes.

Thanks for your quick answers.
We will try the rules for incoming.
I'll let you know if it's work or not.

Greetings Basti

Sometimes the solution worked, sometimes not.
But I don't no why.
I created a rule like:
If "From [!] @" than answer to sender
For testing I send a Messeage from another internal Account and it works.
I  get the automatic Response, but next time, maybe ten minutes later i don't get the response.
Do know what I did wrong

Basti

Oops,  forgot to mention.   GroupWise has a protection to prevent mail loops.  If your auto response goes to someone who just turned on a vacation auto response you could have mail constantly auto reply.  By default a rule will only email the same user one time until the rule is stopped and started or saved again.
I don't remember where it is set (somewhere in console 1) and do not have the time to look right now.  Unless Bill can respond, I will need to research it later.

thanks so much for your help,
now it works perfect.
Greetings Basti