I have just gone through the list of filter options on the btinternet.com server, and I haven't found any rule that says move everything to Inbox.
The best option seems to be something like "If body" "contains" a '.' (fullstop) then "Move to" "Inbox". I suspect very few emails do not include a fullstop in the body. I tried with a space on its own, but that isn't accepted. I don't know whether this will work reliably. I will have to set up such a rule on my various email addresses and see if they miss any emails with a fullstop in them.
Ho ho, this is just TOO much! I can hardly type for laughing.
The latest email to be sucked into my Spam folder is the one from no-reply@community-mail.bt.com telling me that "azoic (Aspiring Contributor) posted a new reply in General email queries in the BT Community:" on this subject.
Well, at least they're not biased but, come on, surely this is testament to how broken the algorithm is and how ridiculous putting them in the so called Spam folder is.
[postscript edit]
... and the next one, hot in, is from the same email address saying "[my] new rank is Aspiring Contributor". What's there not to love!!!!
I use 'If To' contains '@', move to Inbox. This picks up virtually all emails. To cover all possibilities, I have a second rule 'If To' does not contain '@' move to Inbox. This worked perfectly for over 10 years but for some reason has stopped working. I think the rules system had an update and this was broken.