Outlook 2013 - meeting requests are received by groupmailboxes whom they arent send to

[Riesj77]

Goodafternoon

I have a pretty weird issue atm.

A client of ours has a few department mail-only mailboxes.
On these mailboxes permissions are set for a few users who can do basically everything except delete stuff.
This is not really relevant to the issue to my knowledge, but better have to much info then to few.

The problem:
When someone with these permissions sends a meeting request to someone else who is unrelated to these mailboxes (or even unrelated to the company), the meetings are ALSO send to these department mailboxes. Even weirder, those mailboxes seem to be required!.
So eventhough the sender doesnt add the emailaddresses of these department mailboxes, they are still send there.
So it seems that Outlook/Exchange adds them after the sender presses SEND.

See attachement. The right pulldown menu part are the 2 mailboxes that shouldnt be there (sorry the words are in dutch, but the word VEREIST means REQUIRED).

Does anyone have any idea how i can fix this.. or what is causing it?

Kind regards,
Richard