Claws Mail

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]

Biggest feature : being lightweight enough to run constantly in background. It's customizable and supports with HTML view, provided you dive in its settings.

The installation puts files everywhere, not compact.

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.

After I moved from 32 to 64 bit Windows 8.1, the most important Thunderbird addon Mail Alert stopped working. I started looking for a replacement. Tried this Claws Mail. It looked promising as I could make alerts with its Execute function in filters + AHK MsgBox function. But it is terrible with HTML. I found no way to insert pictures into messages, so I'll have to keep looking for something else.

I was using Postbox on a machine with 8gig RAM and an Intel(R) Core(TM) i5-4278U CPU @ 2.60GHz, 2601 Mhz, 2 Core(s) with 4 Logical Processors. Postbox would use huge amounts of memory and CPU time. Switched to Claws Mail and the difference was immediately very noticeable. Postbox slowed down the whole machine. One the other hand, Claws Mail's footprint was barely noticeable. I've been using Claws Mail now for two days, and I've found the learning curve to be not very steep at all. As with all software it has its own little peculiarities, but I found them for the most part charming rather than aggravating. I think that this program will grow on me!

Does not support auto-configuration for IMAP-email-accounts. I will stick with Mozilla-Thunderbird for now.