User can read e-mails from another mailbox

Hello,
I just get email, that one of our users can read emails that are sent to different user. He doesnt have full access, any rights to read his emails, no alias, no forwarding. Hes using Aqua mail for, he gets a message that hes not authorized for 10 minutes but after he can read foreign e-mails. when he installs Outllook for Android, he can only read his emails (thats correct). Why he can read emails, which are sent to another recipient. That's a big problem. Any idea?

Thanks