Some more updates and (hopefully) a solution to the problem.
This may be of use if someone has similar issues…
First of all, today I discovered that Gmail does not move received loopback messages sent from within Gmail itself into the Inbox. Instead you have to select 'All Mail' to see them. This confused the results of some of my previous tests.
Also, yesterday I phoned IONOS support to ask how to forward emails sent to <myname>@<mydomain>.co.uk to my Plusnet mailbox.
I thought I had originally configured IONOS to forward messages sent to <myname>@<mydomain>.co.uk to <myname>@<mydomain>.plus.com. However, it transpires that the email forwarding configuration in the IONOS control panel actually has NO EFFECT WHATSOEVER! This is because the domain is hosted by Plusnet; IONOS are only responsible for domain registration. It turns out that ALL emails sent to <anyname>@<mydomain>.co.uk are directed to <anyname>@<mydomain>.plus.com by Plusnet.
Because of this, my attempt yesterday to forward emails sent to <myname>@<mydomain>.co.uk to <myname>_plus@<mydomain>.plus.com and to redirect emails from there to <myname>@gmail.com would not work. It would only work if I sent emails to <myname>_plus@<mydomain>.co.uk. That’s clearly of no use if you have an existing email address already registered with dozens of third parties. i.e. <myname>@<mydomain>.co.uk.
With the current configuration, emails sent to <myname>@<mydomain>.co.uk will be directed to the mailbox <myname>@<mydomain>.plus.com.
To prepare for when Gmail no longer supports fetching emails using POP, I tried deleting the <myname>@<mydomain>.plus.com mailbox and replacing it with a 'redirect' (using the same name). The redirect points to <myname>@gmail.com.
Now that I am aware of loopback messages not appearing in the Gmail Inbox, I was able to confirm that things now work as expected.
Incidentally, My recent tests indicate Gmail currently does still support fetching messages using POP, despite the warning message I received indicating that feature is about to be removed. I don't know when that will actually happen.
PS – users need to be aware that if you delete an existing mailbox and recreate it, then the password requirements have changed so you may not be able to use the original password.
