This is more like a note to myself for if I ever get a new device, but I have had lots of problems since moving my email to a VPS server hosted by Webfusion in the UK. The primary issue is that the mail server that is installed as part of the Parallels Plesk Panel has an unusual quirk when trying to delete email from the server if you are using IMAP rather than POP on your Apple device.
“Unable to delete message from server” or similar error when deleting. Thankfully the solution is easy!
In the iPhone / iPad settings simply go to mail, and then the mail account and then advanced and in the IMAP Path Prefix add “INBOX” in capitals.