You can manually delete it from sql.

Check the Messages and notifications area.

In addition, you can do this by creating a plugin and using cron. (Recommended)

Do not erase them manually because you have the tables full of dead records and the orphan attachments.

I have a plugin that does this, it is currently in tests on my website.

If you want to do it manually, follow these steps:

1- decide from which date, for example 01/01/2019

2- pass this date to UNIX format, and many websites, for example https://www.epochconverter.com/

3- point the value in seconds, (not in milliseconds), in this example 1546304461

4- Go to your database and execute this:

DELETE FROM `ow_mailbox_message` WHERE` timeStamp` <1546304461

DELETE FROM `ow_mailbox_conversation` WHERE` createStamp` <1546304461

5- Then you go with the file manager of your server to this folder:

misite/ow_userfiles/plugins/mailbox/attachments/

6- Sort by date and delete all previous to that date

With this you will have dead records in other tables, but you will empty the two main ones.

I've used it for years, until I made the plugin that does it the right way, but until I have it tested, have the language file, installer and uninstaller and all this can not be put on sale