My clients don't send spam. They are legitimate businesses sending product updates, nurture sequences, promotions, information, transactional messages, etc. The idea that emails are "spam" simply because they land in the spam folder is incorrect.
I hope you realize that what you just wrote could easily be read as: "They are legitimate businesses sending spam, spam, spam, spam, transactional messages, etc"
Just because you have one valid use case to send an email does not mean that anything you send is not spam.
I can understand this being your viewpoint. Nevertheless, I disagree with you.
I use Fastmail instead of Gmail or Outlook, but as far as I'm concerned, everything in your list but transactional messages is spam to me.
I don't want to hear anything from a business until I've bought something from them. When I make a purchase, all I want is my receipt, a shipping notification with a tracking number, and a reminder that the warranty period has expired and that all data pertaining me has therefore been expunged from their database.
If I cared about product updates, promotions, or information, I would visit the website; that's probably how I learned about the business and decided to make a purchase in the first place. Nor do I tolerate "nurture sequences". If I make a purchase at all, it will be before the business has sent me a single email. If a business intrudes on my solitude before I've decided to do business with them, there is nothing they can do to build trust with me.
Anything that comes from variations on no-reply@ or contains "notification" or "unsubscribe" in the body goes directly to trash and gets auto-deleted after 30 days. (Just in case there's a product recall.)
Why? Simple: My inbox, my rules. I decide for myself what is relevant to me. Not you. Not your clients. Not even God Herself.
I alone decide for myself.
If businesses can't accept that, I will dance on their graves. None of them are entitled to my attention or a moment of my time. I have no love for Google or Microsoft, but I'm always up for a rousing game of "Let's you and him fight."
TL;DR: your fundamental problem is that you stuck your head into Hume's guillotine. You're trying to derive ought from is, where the "is" is that your clients are trying to market by email, and your "ought" is that the rest of us should care that the monopsony resulting from the dominance of Gmail and Outlook makes things harder for your clients. You haven't persuaded me that I should care. Furthermore, your clients do not see me as a human being, but merely a means to their ends. I will not apologize for resenting that.