I believe protonmail has their business plan which probably fits the bill. It seems to check all of your boxes, admin console, custom domains, prexisting and FOSS apps on respective app stores, integration with 3rd party clients like Thunderbird, individual user accounts. For white label, I don't know what that means, it doesn't say that word for word on their site, could be some other name or they don't have it?

https://proton.me/business/plans

Tuta may be ticking all your 5 entries. You can simply test it, sign up, pay the 12 Euros per year, go for custom domain and check the options.

I can’t suggest anyone because I use gsuite legacy but in 2022 when they were going to stupidly start charging for something they said would be free forever people put together a spreadsheet comparing providers. Might be out of date for some but might be helpful.

Here is the Reddit thread for it and Here’s a direct link to the spreadsheet

I switched from hosting my own mail for close to 10 years to using Fastmail. The webmail client is much better than Rainloop and Roundcube. I like it better than Gmail TBH. I’m very happy with my decision, but I’m only hosting for me.

I also considered Proton, but it doesn’t satisfy a couple of your requirements. It is a good value though, and the privacy features are tops.

The best value I’ve seen by far is Microsoft 365. The webmail for Outlook is quite good and you can host your own client if you want and connect with IMAP. It’s especially a good value if you use the family plan. Bonus is you also get MS Office and a TB of storage. Downside: Microsoft. But 2024 MS isn’t the same as 2005 M$.

As someone else already said, I’d suggest FastMail. I switched my personal domain email to them recently and they’ve been great.

Check out Fastmail. Pretty much ticks all the boxes, I've been using them for a decade or more. Ultra reliable, good value and constantly innovative and adding features especially in the collab space.

My only gripe with them is their mobile app is online only in that it doesn't work in a cached offline fashion. That can be addressed through alternative clients though and their web app is pretty slick.

Bonus points for them, you can host your DNS with them too and you can manage it all fully integrated.

Edit: typos

I made the move from gsuite to Zoho mail, and wish I hadn’t.

I notice incoming mail delays several times a year. Support hasn’t been super helpful on that or other issues — I ask to provide or review logs and I get a run around.

Their support of exchange active sync is half baked in my experience, too.

As a pro, it’s reasonably priced and the native interface is ok. You can trial them for free, but don’t expect to catch all the cons in that period.

May I ask what's unmodern about Migadu? Their fast and readable UI is the reason I use them.

Can MXRoute do what you want? Sorry, I used them for a while but I don’t know if they meet your requirements.

I've been using OVH for a while, the price is decent (~1.5€/inbox + VAT) and they have a quite comprehensive mamagement interface, with the ability to create different admins account I believe. Of course you can configure your accounts with any client and the documentation is a bit confusing but complete

I use Runbox, and have for many years. Overall I've been happy with them. I think they have the features you are looking for.

I recommend Opalstack.

How about a selfhosted mailserver with IMAP / SMTP?