workflow mailer in non-prod instance

We are using ebs version 12.2.4

In our non-prod instance usually we have turned off the workflow mailer so that the non-prod instance will not accidentally sent email to customers or suppliers.

Now, we want to test some email in non-prod instance. I have updated the workflow mailer mailbox detail. Before turn the workflow mailer back on, which backend table data that I can update to make sure the status of all the emails has been set to sent or something so that when I turn on the workflow mailer, it will not trigger the system to sent whole bunch of emails out? Please advise. Thank you.