This not a new problem, I think it started with Virtualmin 8.
When creating a new message, every time you type a bit of content, it flashes at the top 'Draft saving" then Draft saved" then goes back to “New Message”.
Each time it updates the Save, the previous save gets moved to the Trash folder, with the end result that the Trash folder gets full of these Saves.
Pretty much makes the Trash folder useless if it’s being used as a place of last resort for deleted email and is now cluttered with all these draft saves.
While writing this I created a new mail in Usermin, typed in 3 sentences - one at a time, and now have 7 new useless mails in the Trash folder. I hate to think how many would be added if I wrote a lengthy email - or reply.
It would make more sense to just delete the draft saves or at least remove them from the Trash when the new email is complete or discarded.
The above client now has 660 messages in Trash, mostly these temp files.
One email that was written has 39.
In a few days the Trash folder is now up to 85.85 MiB
The Inbox is up to 96.72Mib since May 2023.
not sure if this is relevant as i am on Debian or Ubuntu and Usermin (2.521) Webmin Pro/GPL
but Trash is ok, so perhaps specific to your OS? or the way temp files are now being dealt with. but I am sure that the powers that be (staff) are aware