A while back Mozilla announced that they were migrating development to an independent team. I don't know much about that, but my impression as a user is that quality hasn't been affected and it might even be getting more love. I was happy to donate some money to the project last time they did a fundraiser.
The traditional whine: "But everybody uses HTML!"
text/html; lynx -dump %s; copiousoutput; nametemplate=%s.html
auto_view text/html application/x-gunzip text/calendar
alternative_order text/plain text text/html application/postscript
mutt can be used as a pure IMAP client or to read local mail delivered to mboxes or maildirs.
If you are the sort of person who builds up 250,000 messages on your IMAP server, guess what: mutt handles it better than any other client.
mutt is not point and click. mutt is usable by people who can read, and is powerful for people who can spend twenty minutes reading the docs.
I'm the happiest I've ever been with email using that combination.
I've explained the basics of my setup before:
(Granted, I'm a heavy Emacs user, so I'm predisposed to like writing emails in Emacs.)
I previously used elm, mutt, sup, and probably a few other clients, but none of them felt as configurable and complete as my current set-up.
The calendar interfaces with your Google calendar too (with the Lightning and Provider for Google Calendar add-ons).
Available for Mac, Windows and Linux.
It has occasional quirks, but very stable.
Also, worth noting, most of these clients cannot handle Gmail's style of label handling.
If you feel like dealing with terminal based and text only email, I'm sure Mutt is fine. I'm not sure this is realistic for most people.
Evolution is a decent client with very minimal, basic features. It does allow handling of multiple email accounts but it didn't do a very good job of it. There are no plugins. It was often slow to perform actions like archiving or deleting (as if it did them in real time rather than in the background), which meant I couldn't quickly go through and process a list of emails. I had to pause between each action.
Geary was somewhat like Evolution but even more minimal. It has almost zero configuration options and no plugins. It's a very good looking email client, so if you have simple requirements and like the aesthetic it might be a good option.
Thunderbird was the most promising. A rich plugin ecosystem, better handling of multiple accounts. But it was buggy, a memory hog (6 GB RAM and 25% of my CPU when idling), and it would crash on me. There seems to be some history behind it that I didn't get into, but many plugins were not compatible with my version of Thunderbird. For example, I couldn't use the calendar because my version was too new. If critical plugins are going to be disabled any time I update the app, well, that's not worth dealing with to me.
Claws Mail was okay, even if dated looking. It had no contacts or calendar support. There were a few plugins but nothing like what Thunderbird offered. It had a few bugs, like when I switched folders it would scroll to random messages, and overall was too limited for my usage.
Mailspring is the best open source option I discovered. It's somewhat slow at times and can be buggy, but overall performed better for me than Thunderbird. It has some advanced features built in, handles Gmail labels perfectly, and is very clean looking. The biggest downside is that 1) it requires a Mailspring account (though it doesn't send your emails through Mailspring) and 2) is based off a monthly subscription (with a free option). In other words, you're generally dependent on the continuation of the Mailspring service for ongoing operation of your email client.
If open source is not a strict requirement and you are on Windows, Mailbird is the best email client I've ever used. It nails almost everything out of the box, is lightning fast, and has always been stable. If Mailspring doesn't work out for me I'm likely going to switch to using Mailbird in Wine.
Mailspring is not opensource, only the UI is.