Shared mailbox has no history of anything in the Sent archive?

Hello,

I created a shared mailbox a few months ago and gave access to 3 other users. They also use a third party program to send messages as the shared mailbox(in the software itself). Now when I look at the Sent archive of that shared mailbox, there is nothing in it(it would appear as though as if there wasn't a single sent message there).

Is there a way to check that emails are being sent from this shared mailbox?

Thanks,

EDADVC