Hi Lawrence, I have just followed your instructions....
I have 3 POP accounts and an IMAP account configured on my iPhone 6S+.
I do not currently have any emails sitting in the INBOX of one of the POP accounts so I deleted the account, rebooted, re-configured (added) it and then re-booted.
I have re-tested.
These are my exact instructions so if anything at all varies from your device we are not testing under the same configuration....
Click to open mail app.
Click new mail icon in bottom right-hand corner.
Cursor is flashing in "To" field. In this field enter the email address of the account which you have just configured above.
Click in the "Subject" field and enter "Test".
Click in the main email body and enter "This is a test email".
Click Send.
(I don't believe it matters which email address the email is sent from).
I now see my "mailboxes" screen. I have it configured so it shows from top to bottom.....
All inboxes, All drafts, All sent, All trash.
In the same screen and below the 4 rows described above I see the 4 email accounts listed under "accounts".
Click All sent
Top row shows the email I just sent. Swipe left to send it to trash bin.
Click Mailboxes in top left-hand corner to take you back to previous screen.
Click All trash
Swipe left on the email you have just sent to the trash.
We have just done that so there is no longer any record of the email we just sent which could confuse matters.
Click Mailboxes in top left-hand corner to take you back to previous screen.
Click All inboxes. This forces the app to download any emails waiting to be delivered (my accounts are configured to manually fetch)
The email arrives and I can see it at the top of my All inboxes screen which I am now on.
The subject is correct ("Test") and the body is correct ("This is a test email") (I have it configured to preview 3 lines).
Swipe left on that email to send it to the trash bin.
Click Mailboxes in top left-hand corner to take you back to previous screen.
Click All trash.
Now in the All trash screen I can see the email at the top.
The subject is correct but the body now reads "This message has no content" ISSUE/BUG! (I have it configured to preview 3 lines).
Click on the email.
The email body now reads "This message has not been downloaded from the server" ISSUE/BUG!
Click the folder at the bottom to move the email.
Click Sent.
Click All trash
Click Mailboxes.
Click All sent.
Now before I deleted and re-configured the account as per your instructions, the email I just moved from the trash box to the sent box completely disappears off of the device at this stage (is not in the sent box and not in the trash box).
But now having deleted and re-configured the account the email is now sitting in the sent box but still reads "This message has not been downloaded from the server" when clicked.
See how you get on.