Claws Mail (opens in new tab) is an open source email client with more bite than most. It's not quite as slick as rivals like Mailbird Free, but whatever it lacks in polish it makes up for in speed and flexibility. It's made primarily for Linux, but there's also a port for Windows users, with an experimental 64-bit version.
Download here: Claws Mail (opens in new tab)
Developer: The Claws Mail Team (opens in new tab)
Operating system: Windows, Linux
Claws Mail is packed with tools to help you tame even the most unwieldy of inboxes, including quick importing of contacts, a fast message cache system, the ability to download all attachments in an email at once, and a powerful quick search tool.
That's not all; Claws Mail is an open source project, so anyone can use its code to make plugins to expand its capabilities. Many users have done just that, and spent their free time building include tools that you never realised you needed, like one that strips attachments out of emails automatically, and another that alerts you to new emails by illuminating lights on your laptop.
Setting up your accounts in Claws Mail isn't quite as straightforward as it is in Mailbird Free (which just requests your webmail login details), but it's not excessively complicated.
Importantly, Claws Mail doesn't place a limit on the number of email accounts you can add, making it a great choice for anyone with lots of personal and work inboxes to manage.
Claws Mail looks simple at first, and the basic features are all self-explanatory, but spend a little time delving into its menus and you'll find dozens of powerful timesaving tools like advanced message filtering. You can create your own filters from scratch, including any combination of flag color, sender, age, phrase and many more. Custom filters can be applied globally to all messages, or limited to specific folders.
Another of Claws Mail's best features is threat-scanning courtesy of open source antivirus engine ClamAV (opens in new tab) via the Clam AntiVirus (opens in new tab)plugin. Emails are one of the most common vectors for malicious software, so this is a very welcome extra layer of protection for your inbox that won't conflict with your existing antivirus software.
It's a shame there's no calendar tool as standard, and its archive of add-ons can't compete with the hundreds available for Mozilla Thunderbird, but if reaching inbox zero is your goal, Claws Mail is the tool for you.