1. Fastmail: tried and stopped using because of Australian law that is relatively more hostile to privacy and is more expensive that Runbox when adjusted to features (e.g. alias count limit for personal domains)
2. Mailbox.org: Tried and stopped using because firstly they made a payment routing mistake when I upgraded to a paid account during the trial and blocked sending of emails. Took a few days to get it cleared. Also, there are important features you can't try on a free trial like sensing emails to non-mailbox.org domains, configuring your own domains, etc. They quote alleviating spam account risk but it doesn't feel right when runbox allows you to try these features for free before signing up. Also, mailbox.org forces cloud drive, video calls, etc on you. I don't need that crap. Just need a solid mail service. If you want encryption, you have to upload/let them generate the private key for you. While this is technically necessary to encrypt your mail box using pgp, I don't think this is a good idea. I think entrusting the key and the lock to the same people defeats the purpose of encryption.
3. Posteo: doesn't allow personal domains. This is a deal breaker for me.
4. Soverin: I ran into some issue with Soverin. Can't remember what that was. I really wanted to use them otherwise since they seem like a small team and I like supporting small businesses.
Finally I settled for runbox because they show a decent level of commitment to privacy, cheaper for the set of features they offer (unlimited aliases on your own domain, catch all, etc), let you try all their features on free account which I think is a very good sales posture, there's a fantastic deal right now that gives you a two year subscription for the price of one.
I do not have any affiliation with runbox btw. I highly recommend using your own domain in case you need to lift and shift at some point, use pgp keys for privacy whenever possible and take a principled stand against bullshit laws like those in Australia if you care about privacy. Overall, right now, runbox will give you the best set of features and comparable privacy guarantees to their competitors for a better price.