FastMail supports Roundcube Next development

For example:

it pretends to be a gmail style email client where the unit of manipulation is a conversation, not a message. But the underlying message orientation peeks through in certain cases.

In other ways, like settings, it feels very half assed: routing rules have to be very simple and sometimes don't work. The UI for setting up routing rules is shit; you have to add them, then scroll to the top of a very long page and click "apply all changes" for the rules to take (yes, I missed that while porting rules from my old webmail and had to redo 40+ rules). The rules don't work as you would expect: eg messages from "a@b.com" do not match "sender ends with" "b.com". Rules can only filter on one thing at a time -- no compound rules on eg sender and subject. When you create a rule, it doesn't offer to apply to existing messages in the inbox.

When you mark something as not spam, it is delivered to your inbox and skips rules.

It sometimes loses the send button while composing messages.

No option to "filter emails like this"; instead, you have to copy and paste eg the address you want to filter into a screen 3 clicks away.

By default, it doesn't load images in html email. There is a link that tries to load the images in the email you're viewing; it works perhaps 2/3 of the time.

The rich editor is crap. For just one of a long list: paste tsv data in there; it strips all the tabs. Awesome. So a\tb\tc pastes as abc.

Etc. There's just a lot of annoyances that make me assume the devs don't use their own product or they'd fix it out of sheer annoyance.

That said, I pay for it over gmail for privacy.

Oh, and one more complaint: they charge you for sms -- 0.12 each! -- if you setup 2fa to txt a code to your phone, even on $40/year accounts. That's just chintzy. Better yet, because they're run by cheap dicks, purchased sms credits expire after a year!!! When I saw that it felt like purchasing a prepaid cellphone at a gas station level cheap. They're seriously pricing at 1600 hundred times the pricing twilio has on their web page for joe-random-user, not someone sending a million of these or whatever a day.