Deleted POP emails have started to reappear on iPad2

Model MC770X/A

Software iOS 9.3.5 - Your software is up to date

I checked the Rubbish folder just now and it was empty.

I then deleted all 5 emails from the inbox.

On checking the Rubbish folder again it had all emails deleted since I reset the iPad.

May or may not be relevant to the issue...

I'm finding that selecting the Mailboxes option when checking the status of Inbox and Rubbish after going through a delete action is frequently causing the mail app to hang.

I've tried leaving in that state and it doesn't change.

The only way I've been able to resolve is to use the Home button to display the running apps and then close the mail app.

Following that navigation is normal.

All emails deleted since the device was reset continue to come back following a Delete All action in Rubbish but not every time.

To avoid any misunderstanding...

I have 3 email devices:-

1) iPad2      with iOS 9.3.5 which is the latest available for the device

2) iPhone7 with iOS 13.3.1 which again is the latest available for the device

3) Windows desktop

Mail on the Apple devices is only used to briefly screen new emails when away from the desktop - hence the POP setup.

Typically I would read emails on those devices responding only if absolutely necessary and when doing so would BCC myself so my response ends up on the server and is available to the desktop.

I would then Delete All from Inbox and Sent, open Rubbish and Delete All there also.

Both Apple devices are configured to Never Delete from server.

Both Apple devices are configured to Remove Deleted Messages after 1 Day although I always remove manually.

Mail management is carried out on the desktop where the configuration uses IMAP.

There I read all server emails, move to local folders on the desktop if I need to retain and then clear the server by deleting anything left and clearing the Deleted folder.

I check all IMAP folders at least daily to make sure they have been cleared.

In practice I would never expect to have more than a half dozen emails on the server at any point in time.

I have verified the server status using Webmail connectivity and am certain that the mail client on my desktop correctly reflects the server.

I use all 3 devices on a typical day and make sure they are as up to date as possible.

The issue only presents on the iPad device although the iPhone is configured in the same way and mail handling is identical on both.

Following a reset of the iPad the issue remains with all emails deleted on the device since the reset reappearing on at least a daily basis.

The reappearance of previously deleted emails seems to be triggered by the deletion of emails from Inbox - I check Rubbish before clearing Inbox to confirm it is empty.

Deleted emails do not reappear every time I delete from Inbox but when they do all emails deleted on the iPad since the reset seem to be included.