Claws Mail is a simple, super lightweight email application. If all you want is email and RSS, Claws Mail is great. It takes up very little memory and does those basic tasks very well. It also supports minimization to tray (in both WIndows and Linux), making it a rarity among open-source email clients (and giving it a distinct advantage over Mozilla Thunderbird.)

Why four stars instead of five? A couple of idiosyncrasies that are either acceptable or enough of a deal-breaker that you'll want another email client. Claws Mail doesn't support CardDAV, but if it's the primary email application you use, you can synchronize vCards and import them to each system that you use Claws Mail on. It's not ideal but it works. Claw Mail also doesn't support IMAP IDLE, so you'll have to accept email checking at regular intervals (I set it to five minutes) rather than immediate notification upon receiving a new email.

The default theme is also very ugly, but Claws Mail has support for custom themes, so this can be fixed. I recommend the Elementary theme on Windows and the Breeze theme on KDE in Linux.

[Edited by bVork, October 22]

On windows 10: Fetches and reads mail ok. Freezes on attempting to send an email. No crash until the system prunes the process so no error message. Tried troubleshooting firewall to no avail. Couldn't find same issue in forums. Gave up.

I keep Claws-Mail 4.00 installed on my PC (Linux Mint) but I have a big trouble with the colums (From, Subject, date...). They are too much larges and I can't really straighten them with the mouse. And the columns headers don't show the words (From, Subject, etc). I have to click in the place were those words sould be but that makes much trouble in those colums. Perhaps this trouble began 4 or 6 years ago, I'm not sure.

Much faster than thunderbird.

It also uses around 12-40 mb most of the time, whereas Thunderbird with chat disabled, no plugins, no themes, and no calendars uses around 350mb, sometimes jumping into the thousands.

Setting up mail is a one time thing, though you have to do more manually than Thunderbird. It's pretty easy though, even for 2-factor authentication. Just go to your mail and get the app password.

A lot of mods are only on Linux. The only thing I really want on Windows is dark mode.

The other plugins work well, like being able to close to tray (whereas on Thunderbird at the moment I have to minimize to tray).

Not the friendliest of applications. I've tried it on and off over many years and it's never really changed; especially the interface which is somewhat dated.

Doesn't autofill server information very well so if you don't understand how mail servers and email client back-ends work you might want to try something like Thunderbird or Evolution (my favourite on *nix), particularly if you just want to hook up some Gmail accounts.

Very easy to create accounts, and the user interface is very tidy and clean. Claws Mail has also an address book. All in all, more lightweight than Thunderbird, but the downside of Claws is that there is no calendar included.

Hands down the best Email client out there. No irrelevant features, unlike Thunderbird. Handles email communication nicely and has everything you may need. Supports PGP encryption on all platforms! Love it.