Email with no content - Won't download, can't delete, can't move!

Morning all,

Got a bit of an issue with an email. The day it arrived, I opened it, read it and then moved it into an appropriate folder.

Some time later I opened mail again and I got a message giving two options, 'Undo' and 'Cancel'. I hit Undo not knowing what it was, or if I'd accidentally requested something thinking that Undo would both cancel the operation and put whatever it was back.

From that, the email I had previously moved reappeared back in my inbox, however the email no longer displays any content and won't download. After a minute of so it displays "This message has not been downloaded from the server"

If I delete the email, it goes for round five minutes, then I get told it cannot be deleted. If I move it, the same thing, five minutes later get told it cannot be moved. So I can't delete, can't move, and it won't download. What's going on and how do I fix it?

I've tried all the standard stuff bar one, and that's the deletion of the email account. The reason I haven't deleted is there's conflicting information regarding what happens with any folders I've created in the account. Some posts say if you delete the account, the folders get deleted but all emails go into the inbox by chronological order. Other posts say everything gets preserved. The worst says that if you don't backup and you delete, everything gets deleted, end of.

Clearly I cannot try this method until the outcome is corroborated.

I want to keep everything, including all my folders, structure and layout.

Any ideas why this is happening, how I can resolve or can shed some light on the email account deletion query?

For reference if running iOS 11.2.5

Kind regards

Mitch