But are there any good (modern) web clients (see Outlook Web)? E-Mail, in my experience as a light user, involves a lot of client side configuration (filters, sender names, ...) that I can't stand being tied to a single client device.
IO.OX (used at mailbox.org and strato, among others) is "alright", but I haven't seen much material about self hosting it.
I use thunderbird and iOS mail. With thunderbird, configuring an account is trivial with mox using autoconfig. Needs just an email address and password, the config is found automatically. I haven't gotten auto configuration to work with iOS/macOS mail or Outlook (with thunderbird-style autoconfig, microsoft-style autodiscover, or SRV-record style). Don't know what I'm doing wrong. I've seen apple products make requests to apple servers when trying to autoconfigure an account, but no requests from apple to me. I presume you they have some kind of allowlist. From Outlook I get some connection attempts from Microsoft, but they can't seem to set up a TLS connection (connection setup fails during negotiating; I've also seen them send invalid SNI names (trailing dots)). This shouldn't be hard... If anyone knows how to autoconfigure accounts with the mail clients that most of the world uses, let me know. (:
I'm wondering what kind of filters and settings you would configure on the client side. I would think you would do most on the server side. That doesn't mean it has to be integrated into a web client. Mox has an account web page where you can configure mailbox routing.
Once more urgent features/omissions are taken care of, I want to add a web mail client to mox (optional of course). A basic one should be doable.
The Dovecot instance has Sieve available to it, and Roundcube has the sieve plugin installed and able to write server-side rules for filtering. Address book is stored in a DB, and exposed via nextcloud's carddav. Works in KMail, iOS mail, Thunderbird.
I dunno if you can consider Roundcube to be modern, but all I want from a mail client is the ability to read and compose email, so it works for my needs.
Now, those mailbox.org and others use (probably) commercial version. which comes with additional functionality (like document converters for preview) and providers make their own customizations top of that anyway.