Really like tuta, have to admit that i've not looked closely at the paid options but they provide their services for free to our open source project and it functionality even on that tier is pretty great.
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?
I canโt 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 Might be out of date for some but might be helpful.
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. Iโm very happy with my decision, but Iโm only hosting for me.
I also considered Proton, but it doesnโt satisfy a couple of your requirements. It is a good value though, and the privacy features are tops.
The best value Iโve 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. Itโs 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 isnโt the same as 2005 M$.
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.
I made the move from gsuite to Zoho mail, and wish I hadnโt.
I notice incoming mail delays several times a year. Support hasnโt been super helpful on that or other issues โ 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, itโs reasonably priced and the native interface is ok. You can trial them for free, but donโt expect to catch all the cons in that period.
I've been using OVH for a while, the price is decent (~1.5โฌ/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