Hi,
I actually solved the issue.
Just in case it happens to someone else here are the steps.
- Create a new user which you are going to connect the mailbox to.
- Right-click on the user and choose Exchange Tasks.
- Choose Remove Exchange attributes.
- Go on Exchange System Manager, select the mailbox, right-click on it and select reconnect.
- Choose the user you just created and the user will be connected to the mailbox.
Hope this info is will be useful to someone else.