I use posteo.net for 10 years now, and I am super happy with them. I don’t get spam, and I have never missed an email.
They choose not to use a spam folder system, and I understand their reasoning, and agree with them.
They sometimes get criticized for some other decision (something to do with certificates, if I remember correctly), but after reading their reasoning, I agree with them.
In my experience, they have now real downside, and I recommend them to everyone I know when they come to me with email problems.
I had this issue with sponsored websites. It took me a while to figure it out, but this resolved it for me:
When you just disable showing sponsors, Firefox will still autocomplete them.
If this guide is too vague, I can be more explicit.