https://docs.whmcs.com/System_Utilities#Prune_Saved_Emails
forget the wrong info in the docs, you would go to utilities -> system -> system cleanup -> advanced -> prune saved emails.
in reality, all that function is is a delete SQL query to the tblemails database table of emails sent before the entered date - in other words, the SQL query Steven didn't recommend that you do. 🙂
that said, I would backup the database, or at least that table, just in case years down the line, you ever need to prove what was sent in a particular incident.