You only have one way to do it. You suspend the user, delete the messages with the Mail Clean Pro plugin
https://owspain.com/store/productview/4
https://developers.oxwall.com/store/item/1702
and then reactivate the user.
The plugin will do it correctly by removing the attached files if there are any.
You cannot do it directly from the database because there are multiple tables involved.
Yes. Create another administrator account, remove your current profile from administration, suspend it, delete the messages and undo the changes by reapproving it and returning the permissions.
I don't recommend fiddling with the message tables of the MailBox plugin, they are quite complicated and you can destroy all the private messages from everyone.
Then you can leave the plugin working for what it was designed for: to eliminate old messages, (for example older than a year or two), and those of users that no longer exist ... You will save space and gain speed. It will also be responsible for eliminating the associated "attachments".