Send an email from a shared mailbox after modifying the default mail address

Hi everyone,

I'm encountering an issue with Exchange 2010.
I use a kind of shared mailbox I access to as an aditionnal mailbox in Outlook (my account has full access permissions on this mailbox).

Before, I was able send an email from the main address of this mailbox after modifying the field From.
But yesterday, I added a new address to this mailbox and I set it as default address (using the button Set as reply in Exchange console).
And I can't send mail using the new default mail address, the old one still works. I have a message that says I'm not allowed to send as other people.

Do I have to modify a specifi setting ?

Thanks in advance for your help.