Is Zoho not mainstream? I thought they do business email mostly.

I use protonmail. One possible downside is you can't use an external mail client, but I do not make use of this feature.

I have heard good things about PosteoMail

It's not popular here in Mexico! so at least here, it's not mainstream (but people mostly use gmail and sometimes outlook/hotmail addresses, rarely iCloud ones). It probably is more frequent in other places.

I would advise staying away from Tutanota, or Tuta, as they're called now. I have been a paying customer of theirs for 3 years now and the service is not very good, while I have never felt like they were deliberately providing poor service or anything like that, and they are definitely a cut above google or other "free" providers, their applications are all terrible, and improving at a snail's pace.

No proper email search (enjoy letting mails index for 20 minutes everytime you want to search for something, oh, and resetting the search date range params everytime too), very clunky, bloated and inefficient interfaces that just aren't nice to use for anything other than tapping on mails, poorly implemented dark theme that makes lots of emails hard to read, and a litany of other problems.

Uptime isn't that good either, while not exactly terrible, its been down often enough to cause issues for me, which I can honestly say has never happened with any other provider.

This next one is more of a trade off of the type of service than anything they're doing wrong, but IMO its a trade off that isn't worth it given email's inherently security averse nature: because the service is built around encrypted email, you cannot use another email client like thunderbird to get around the UI, which would be a fair trade, until you realise that 99% of emails just aren't going to be sent E2E, emails sent E2E can be forwarded on non encrypted by accident very easily and for those times that you do use encryption, you can use PGP with other services easily. (PGP isn't all that good either BTW)

I'm looking for a new provider at the minute and am seriously thinking about running my own mail server on my VPS, I have an email server up and running for my website already, but that approach has got its own issues with getting caught in spam filters and stuff.

I wouldn't send anything you think seriously needs to be kept secret over email, ever, use something less full of holes, or use analog comms.