> Saying that they're spam just because you've never received email from them would cause most people to be unable to find these confirmation messages.
It's not saying that they are spam. It's just saying that you never received a message from them. That account confirmation email you are expecting will be obviously marked, but that phishing email claiming to be from your bank will be marked too. You look at the mark and decide what to do.
Email clients probably don't do it because it is not as useful as it sounds. Impersonating email senders is not hard, so phishers will just do it.
> If the email address isn't priorly known, it shows the email address instead of the display name.
The only email client that I have ever seen doing that is the roundcube instance I configured on my VPS. I use several clients, nearly all of them either hide the sender address or decrease its relevance enough so that nobody sees them.
I'm in complete agreement with your comment about spam filtering. The only thing is that somehow, it feels like it worked better at the earlier 00's. Nowadays the training for your account will be dissolved in a huge set of unreleated data, so that anything specific for the spam you are receiving will never be reflected on the filter. That is both for marking things as spam and as not spam.