Thunderbird

I have tried several email clients including Outlook, Em Client, Claws Mail, Zimbra Desktop, Mailbird... and went back to Thunderbird every single time.

Thunderbird is simply still the most customizable email client out there that delivers a solid, stable performance for the unbeatable price of free.

Supports an unlimited number of mailboxes, calendards, has full-text email search, can show you an unified inbox, you can easily compose HTML messages, read RSS feeds, use extensions etc. It's really a Swiss army knife.

The only real alternative is Claws Mail if you rather prefer a fast, lightweight client.

If you only use 1-2 mailboxes (or don't mind paying) and use Windows, Em Client is a decent alternative, too, with a steady release period.

Good to know: Thunderbird is not an abandoned project, it's maintained by the community and bugfixes, minor feature updates are being released periodically.

Thunderbird has always been my favourite email client. But update after updates, Thunderbird gets worse and worse.

On each update, disabled extensions increase, and lot of them are no more updated, or, when they are updated, doesn't work as they should.

Some features asked for years by many users are still not made (like mail summary in list, as you can see in Apple Mail, and most of other email clients). Can't use maildir (beta / unsafe) instead of mbox.

I'm really fed up fighting with Thunderbird on each update with features like "dark mode" I don't really need or could wait for (as they disabled themes support which was useful for "dark mode" or "accessibility").

Looks like Mozilla is trying to kill Thunderbird !