Enabling IMAP readonly and disable SMTP for a specific user

I am going to fire an employee. He has left and we will fire him via email and postal letter.

We know he checks his email (via logs) so we want to give him a copy of his termination letter via email.
We don’t want him to reply to us, other employees or clients.

Hence, we only want to allow him to read his IMAP email but have no SMTP access.

We are running Server 10.5

Any tips on how to do this?

I can always monitor the logs and once his message is read and downloaded, I figure I can just disable the entire account.