Hi Fernando,
That's what we have as well. We did it in this way.
1. Set up groups for your clients. We call them "Client A", "Client B", etc.
2. Setup Customer Users <-> Groups and link the customer users to the groups created in 1
3. Create a queue for the client and set the group to the group created in 1
4. Set the group of the other queues to something else then the group created in 1
Done.
I've found it very helpful to take a piece of paper and create a quick picture of what clients, groups and queues I wanted to have. With that in hand, it was very easy to map it out in OTRS. Maybe it helps for you as well.
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cheers,
arthur