Completely remove an email address and it's ability to send emails

Hi, I deleted some email addresses from our domain but I can still send emails from these deleted email addresses via outlook. How can I delete them completely from the backend? I don’t want them to be able to send email anymore. Ie. Employees that have quit. Maybe it just takes longer to process in the system? I did this over two hours ago. Thanks!

If the outgoing mail server is set to other than your domain mail server i.e., I am not sure that this can be done at all. Of course incoming mail to those addresses is going to bounce.