Today I received an email advertising AT&T services from one of those "suppliers" that are not actually the original provider. So far I cannot delete/move it from my inbox. I am using Apple Mail so that may be part of the problem but this has never been an issue before. Can't even grab it and move it into junk or trash. Hopefully it will go away eventually but afraid it might be a permanent resident. Anyone else experienced such an issue?

I don't use Apple mail, but I'm wondering if it could be an Advertisement? AT&T partnered with Yahoo to deliver its email services - there is a perpetual "Ad" type email at the top of my inbox. I can't delete it or otherwise move it. However it does change ever so often to a new ad...

There are ad blocking extensions made for mail ads.
I use one for my Yahoo mail and it eliminates that ad at the top.
They vary depending on the browser you're using; you have to search for the right one.

I don't remember the name of the one I have on my laptop. I'm on my tablet right now, so I'm unable to look at it.

I looked the post over closely and it was sent to my address and CC'd. Never seen this before. Anyway I took that email address offline and did a drag to trash twice and it finally went away, then deleted from trash. I am making the assumption that being both in my inbox in two ways at the same time may have been what caused this. Weird!

The top line 'Ad' in my att.net (yahoo) Inbox appeared somewhat recently (a couple of months ago?). I've been ignoring it because it's somewhat unobtrusive and seems to be more a link to a Ad than an actual Ad. I use Firefox ESR w/ AdGuard AdBlocker (LOTS of filters enabled) and Brave w/ it's embedded ad blocking, and neither remove that top line 'Ad' in my Inbox. They remove EVERYTHING else, EVERYWHERE else, but that!