New request is created when request is moved to different BU

New request is created when request is moved to different BU

Hi,
 
I'm trying to use multiple BU's for our organization. Our organization is setup like this:
- We have a help desk that assesses all incoming mail. (We made this "DBU"... is that OK?)
- We have several BU's that handle different parts of our customer base
- Help desk moves a request to the appropriate BU after arrival of a new request.
 
It's important to understand that BU's don't have their own external e-mail address, all their conversation takes place using the "general" e-mail address ( support@mycompany.com)
 
Now this happens
- BU (other than helpdesk) sends out an reply to the customer.
- Customer replies to this request (with the hash in the subject line etc)
- The mail is not automatically associated to the original request, but a new one is created in the helpdesk (D)BU.
 
Why is this? Why doesn't it go to the correct BU anyway?

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