I think there is a simple solution for your issue. Use wildcard addressing! So admin should send a mail to *.*.*. Do not forget that admin should have the right to use this kind of wild card addressing (client options)!

Thank you very much (vielen Dank) for your quick answer.
If I understand you right, there is no possibility that the admin see the email addresses in the address book or even see the complete system address book?

Best regards, Uwe.

Uwe,

maybe there is one. And I had some other thoughts too (i.e. distribution lists). But this one is really simple 

Diethmar,

ok, the wildcard solution is simple. But only when writing to all users in PO-B. In GW-Admin-Console there seems not to be some settings for the amin to enable visibility for the system address book. I will look into distribution lists...

Maybe another option is to have a user (dummy user or real one ) in PO-B and have this one share an add. book with the admin(s) that need this on PO-A, the shared add. book has all users in PO-B in them.

The group as Diethmar suggested will work fine as well only then its all or nothing meaning all users in the group, with the shared add. book the admin can also use it for sending mail to only some users if needed

That was my idea too as kind of a workaround. Even better it would be if in GW-Admin-Console there would be a possibility for the GW-Admin to allow (some) users to use address books from other POs.