Other people have mentioned Tutanota; as a user myself I can point out a few pros and cons:

Pro:

• Extremely privacy focused - everything is encrypted and even they don't have access to your email content.
• Pretty cheap for a pro subscription (although they recently changed prices and I'm not sure exactly what the new ones are like).
• Pro subscription you can use custom domains and set up a few aliases.
• Generally works fine.

Con:

• Can only use their custom mail clients (in addition to the webmail interface), because they use their own encryption algorithm.
• Tutanota is so extremely hardcore about security that it seems like a detriment to their user experience. I used to follow the Tutanota community on Reddit and there were so many posts from people who tried to sign up or access their email and were blocked, and they had to go through quite a process to get unblocked. I myself went through a period where I kept being unable to access my account on my phone and it turned out to be that they were blocking me because of "too many IPS connecting to my account from the same IP". I have 2 devices connecting - my PC and my phone, so apparently 2 devices is too dangerous for them. That happened 2 or 3 times for a few days to a week each time, but I haven't experienced it for a while now, though.
• The user interface is not for everyone. I don't mind it myself, but I've seen a good number of people complain that it's too sparse and/or ugly.
• Search is annoying - because everything is encrypted, emails can't be searched on the server side so the clients have to do the searching locally which requires building an index. If you happen to clear your storage you have to reindex everything again. It's also pretty slow and annoying to index further back than a few months.
• You don't get much storage (like, only 1gb or something) and you have to pay to get more.

Overall, if privacy and control over your own email is important to you then Tutanota is a great choice. Just be aware that usability can be kind of a hassle.

Proton mail

When I was doing the same research a few years back Fastmail was recommended to me and I’ve been very happy with it. It was fairly easy to set up with an email address from my own domain too.

In no particular order: Fastmail, Proton, ~~Outlook~~, ~~iCloud~~, ~~Yahoo~~, Gandi (free if you buy a domain), I've heard Hey is ok, but haven't used it.

Tutanota or Protonmail

I'm surprised nobody has mentioned PurelyMail. Its technically in beta, but I've been using it without issue since the beginning of this year (2023).

I use ProtonMail

Check out posteo. It costs €1/month (you can pay more for extra aliases). Seems like a cool company, and you can use it with your own email apps (Thunderbird, K9, etc).

Proton mail!

I use fastmail so I can easily use a desktop client and create aliases with bitwarden and it's a great service the only downside is it's not as private as proton mail, but I don't consider email private or secure

proton mail is pretty decent.
not the most private solution but it's still better

I've been using migadu for almost a year now for mail, tasks, calendars. Works great for me.

I just use IMAP through my domain hosted on Dreamhost (had it for about 13 years now??) and then use K9 and Thunderbird to read it.

Don't need to deal with all the hosting my own mail server spam and ISP nightmare :)

I am happy with Proton.

proton mail (if you wanna exclusively use it pay for it), tutanota and any eco focused one. if you wanna keep using some Gmail accounts use k-9

I'm using Posteo and have absolutely no problem with it. The base price is 1€/month and for my purposes I haven't needed to buy any extra stuff (like extra space or aliases). It also allows access via mail clients by IMAP and POP, which is something I've seen many popular gmail alternatives not providing, despite being IMO a pretty important feature.

You mentioned storage space is important for you. The default size is 2.0 GB, but you can acquire more by paying +0,25€/month for every extra gigabyte up to a maximum total of 20 GB.

It should be noted however that it is a German company and therefore has to comply with German laws.

The one thing I'm not really convinced by is their approach to spam. The web interface doesn't provide any way to define rules to filter out spam except for filter exceptions, but the service already filters out spam for you and it will never reach your inbox. I would normally think that's a bad idea, but I've never received any spam nor have I noticed any mail going missing (except for my lemmy.ml registration mail which I remember I had problems with but I don't remember if it was Posteo's fault and if yes if it was their spam filter in which case it could be allowed to reach your inbox by adding it as an exception).

You can always pay for your own mailserver. I personally like it that people can reach me a hello@firstnamelastname.de. Maybe it is something for you?