Alternatives to Postbox for Linux with any license

Thunderbird

Thunderbird is a free, open source, cross-platform e-mail, news and instant messaging client. The project strategy is modeled after Mozilla Firefox.

Free Open Source Mac Windows Linux BSD PortableApps.com ... Haiku
Extensible by Plugins/Extensions Built-in RSS reader Support for IMAP Multiple Account support ... SMTP integration Built-in Calendar Calendar integration Categories Customizable Delivery reports Extensible by Plugins/Extensions Free API Gmail integration Google Calendar integration Import from Google Reader HTML Compose Integrated Chat Integrated Password Manager Integrated Web Browser Legacy addons support Export to OPML Import feeds from OPML POP3 Support Portable Secure File Sharing Tabbed interface Support for Themes Add a feature

1880 Like
Evolution

Evolution is the official personal information manager and workgroup information management tool for GNOME. It combines e-mail, calendar, address book, and task list...

Free Open Source Linux BSD S/MIME GNOME OpenPGP
CalDAV Calendar integration Built-in RSS reader CardDAV ... Encrypted Email Integrated Spam Protection Multiple languages POP3 Support Add a feature

164 Like
Mailspring

Boost your productivity and send better email with Mailspring, a beautiful, fast email client for Mac, Windows, and Linux. Mailspring is free to use and an optional pro...

Freemium $ $ $ Mac Windows Linux Arch Linux Fedora ... Ubuntu
Multiple Account support Night mode/Dark Theme Unified inbox Customizable ... Email tracking Drag n drop Email Analytics Extensible by Plugins/Extensions Support for IMAP Support for Themes Threaded Conversations Add a feature

50 Like
Claws Mail

Claws Mail is an email client (and news reader), based on GTK+, featuring Quick response Graceful, and sophisticated interface Easy configuration...

Free Open Source Windows Linux BSD
Lightweight Built-in RSS reader Content Filtering Customizable ... Extensible by Plugins/Extensions Support for IMAP Integrated Email Client Multiple languages Multiple Account support Add a feature

83 Like
Geary

Geary is an email application built for GNOME 3. It allows you to read and send email with a simple, modern interface.

Free Open Source Linux Chrome OS BSD GNOME
Multiple Account support Add a feature

74 Like
Mutt

Mutt is a small but very powerful text-based mail client for Unix operating systems. Some of its features are: color support, message threading, MIME support (including...

Free Open Source Mac Windows Linux Xfce
Command line interface Support for IMAP POP3 Support S/MIME support Add a feature

54 Like
Sylpheed

Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA). Sylpheed runs on many systems such as Windows, Linux, BSD, Mac OS X and...

Free Open Source Mac Windows Linux BSD PortableApps.com
Lightweight POP3 Support Portable Add a feature

54 Like
Kontact - KMail

KMail is the email component of Kontact, the integrated personal information manager of KDE. Kontact is a personal information manager and groupware software suite for...

Free Open Source Mac Windows Linux BSD
No features added Add a feature

34 Like
Shift

Shift is an email client that makes navigating between Mail, Calendar, Drive + your favorite Apps & Extensions easy (for Gmail, Outlook.com - Office 365 &...

Freemium $ $ $ Mac Windows Linux
Unified search Multiple Account support Android Sync Cross account search ... Gmail integration Google Apps integration Apps that Sync with Google Calendar Google Drive integration Sync with Google Drive GSuite Integration Multiple Account support Outlook integration Salesforce integration Add a feature

24 Like
Kontact

Kontact is the integrated Personal Information Manager of KDE. Kontact suite unites mature and proven KDE PIM applications under one roof. Thanks to the powerful...

Free Open Source Windows Linux
Convert Email to Task Add a feature

27 Like
Trojitá

Trojitá, is a fast Qt IMAP e-mail client that respects open standards and facilitates modern technologies. Trojitá values vendor-neutrality that IMAP provides...

Free Open Source Windows Linux
IMAP debugging Support for IMAP QT Based Add a feature

12 Like
Zimbra Desktop

The free Zimbra Desktop client allows you to meld the online and offline worlds – storing and synching your email, calendar, contacts, files and documents in the cloud...
- Discontinued From download page: "The Zimbra Desktop is no longer supported. It will reach the end of Technical Guidance October 1, 2019.". See https://www.zimbra.com/downloads/zimbra-desktop/
Free Personal Mac Windows Linux
No features added Add a feature

30 Like
Pantheon Mail

Pantheon Mail is a free open source email client forked by the elementary OS community from Geary...

Free Open Source Linux elementary OS
Multiple Account support Add a feature

8 Like
Nylas Mail Lives

Nylas Mail Lives is a fork of Nylas Mail that fixes many longstanding issues, improves the client experience, and is committed to...
- Discontinued On its github page it states it is archived and is no longer maintained.
Free Open Source Mac Windows Linux Self-Hosted
Extensible by Plugins/Extensions Multiple Account support Add a feature

4 Like
Hiri

Fast, good looking email client. Lighter than MS Outlook with some unique and clever features. Works ONLY with Outlook / Exchange.

Commercial $ $ $ Mac Windows Linux
Calendar integration Office 365 integration Convert Email to Task Add a feature

30 Like