Yeah, me too

There are several open source mail server options available.

The only real new protocol is JMAP which is pretty good and I’d much rather more clients and servers support JMAP than create a new protocol altogether.

I've never seen a piece of software that managed to implement all of the iCalendar specification, which to me seemed like a data model for a good productivity app that's just never manifested. iCalendar (RFC 2445 from 1998) outlined not only events but todo and a journal component for memorializing meetings. Outlook seems to ignore VTODO entries in iCalendar completely, and VJOURNAL support is deprecated.

Since last year, TB has native caldav/carddav support.

I'll stick with Owl for a while longer. This native feature doesn't support calendars or address books, making it essentially worthless to me.

Also it's based on a protocol that's dying (slowly).

Good to see they're attending to exchange support, but this is a lackluster step.

(OFFTOPIC!)

I would not recommend that with any email clients. Most are built with the assumption that you have around 3 to 8 accounts. UI, speed, and configuration may become an issue. Esp. the unified inbox in Thunderbird was slow in my personal use.

What are these mailboxes? Are they changing a lot? That's also a factor to decide in your setup.

If you have hundreds of mailboxes, and you're posting on HN here, chances are you are technically competent. I would recommend a local IMAP server like Dovecot or Stalwart installed as Docker, and then fetchmail or similar to pull (copy) all the mail into a single inbox. And then your email client uses only that one account which has all emails.

Try Thunderbird