Email clients based on Thunderbird
From MozillaZine Knowledge Base
There have been many applications that use the same mail and news library as Thunderbird or are based on Thunderbird. However, most of those projects eventually died off, leaving mainly Linux distro specific builds, Portable Thunderbird, PostBox and SeaMonkey.
- [Discontinued] Beonex Communicator was a sibling of Netscape Communicator. It's email/news client was based on the same code that was later on used by SeaMonkey and Thunderbird. Its of historical interest only. Netscape Communicator 5 was coordinated by Mozilla. It was abandoned (not released) and then rewritten by Mozilla as Mozilla Suite. That was eventually abandoned. Versions 6 and 7 of Netscape Communicator were based on Mozilla Suite. SeaMonkey is a community based continuation of Mozilla Suite.
- [Discontinued] Correo is a email client for OSX based on code from Thunderbird and Camino (a Mozilla OSX browser).
- [Discontinued] Qualcomm announced in October 2006 that they were going to create a open source version based on Thunderbird and would stop selling Eudora when it was available. It is based on Thunderbird 3.0.4 and includes a Penelope extension which can also be used with the normal version of Thunderbird. It was originally called Eudora 8 but was released as Eudora OSE on September 13, 2010. All development has stopped and the support forum has disappeared.
- Apple dropped support for Rosetta in OS X 10.7 (Lion). That means you can't use Eudora OSE to import your old Eudora profile (it is PPC code that requires Rosetta to run) but the rest of Eudora OSE should still work. 
- The last version of the Thunderbird 3.1 branch (3.1.20) was released March 7, 2012 . This means there will be even less support for add-ons that might work with Eudora OSE. Version 1.0b1 is the last version of Lightning that supports Thunderbird 3.0.x.
- [Discontinued] FossaMail is an optimized and enhanced build of Mozilla Thunderbird, developed by Moonchild Productions, the developer of the Pale Moon web browser. It has different defaults (both in the user interface and under the hood), has been rebuilt with a newer compiler and better optimizations than Thunderbird, uses it own profile location (so that it doesn't interfere with existing Thunderbird installations) and keeps using the interface previous to Australis. There are 32-bit (x86) and 64-bit (x64) versions available for Linux and Windows.
- FossaMail/Pale Moon uses the Goanna rendering engine, a old frozen fork of the Gecko rendering engine used by Thunderbird/Firefox/SeaMonkey. Thunderbird (as of January 28, 2016) is currently at version 38.5.1 while Goanna is using the equivalent of Thunderbird 24 so it is behind over 100 security fixes according to Security Advisories for Thunderbird.
- [Active] Interlink is a Mail, RSS, and Newsgroups client based on Thunderbird source code and built on the Unified XUL Platform. It replaces Fossamail, which was discontinued in July 2018. Its available for Windows and Linux, but not OS X. There is no portable version.
- The Unified XUL Platform is a (ESR-52) fork of the Mozilla toolkit code that uses XUL instead of the WebExtensions API. Interlink has very few add-ons and currently doesn't support any of the most popular add-ons such as Lightning, ImportExportTools, Provider for Google Calendar, Manually sort folders, CompactHeader etc.
- Many Linux distros provide a version of Thunderbird that can be updated/installed using their package manager. This may include trunk code (code not released in the current version of Mozilla Thunderbird), some changes in where/how it is installed, or rebranding due to trademark issues. For example, IceDove is a email client distributed by Debian that is based on Thunderbird but removes any proprietary artwork and plugin support. See Moving from Windows to Linux for more information.
- Some Linux distributions have separate repositories for beta releases. Also, if a specific branch version came with your installation, you may continue to receive maintenance updates for the old branch (e.g., TB 1.5 or 2.0) through your regular updates only, and may need to change the repository to get the current branch (TB 2.0 or 3.0 once it is released).
- [Discontinued] Mobility email provided support for running Thunderbird from a USB drive. It was a less known alternative to Portable Thunderbird that provided its own launcher and had built-in support for S/MIME, OpenPGP (Enigmail) and several webmail providers such as Hotmail, Yahoo! and AOL. It appears to have died around 2007.
- [Discontinued] Mozilla Suite combines a browser with a composer, email, newsgroup and chat client, as well as a calendar. (Mozilla split off Thunderbird as a stand-alone email client, and Firefox as a stand-alone browser client.) The suite is maintained as a community project (SeaMonkey) [see "SeaMonkey" below].
Mozilla Thunderbird for U3
- [Discontinued] Mozilla Thunderbird for U3 provided support for running Thunderbird 3 from a USB drive under Windows. However, its domain has been sold and Sandisk only has a version for Thunderbird 18.104.22.168 at Recommended "U3 smart" Programs. The project appears to be dead. The Portable Apps web site has a U3 Apps (Discontinued) forum.
- [Active] Portable Thunderbird provides support for running Thunderbird from a USB drive under Windows. It does not install or register anything on the PC in order to make it more portable. One side effect of that decision is that the profile manager doesn't work. It is basically a copy of the latest released version of Mozilla Thunderbird bundled with a PortableApps.com launcher. Because it reuses a copy of Thunderbird, it is not unusual for users to update it to use the next major version of Thunderbird before it is released or find a web site that has modified it for them.
Portable Thunderbird OSX
- [Discontinued] Portable Thunderbird OSX provides support for running Thunderbird from a USB drive under OSX. It is provided by the Free OpenSource Software Mac User group (FreeSMUG). It uses a platypus script as its launcher. They added support for Thunderbird 3.0.4 on 2010-05-20. It is not clear whether you can use more recent versions of Thunderbird with that launcher.
- [Active] PostBox is a commercial email client from a startup founded by Scott McGregor (the former Thunderbird lead engineer) and Sherman Dickman (a former Director of Product Management at Mozilla Corporation). You have to pay for any updates. It does not have an open source license, a Linux version, or support any languages other than English though there are several community-translated Language Packs. Postbox has much stricter system requirements than Thunderbird and drops support for older versions of operating systems quicker. It can import data and settings from a Thunderbird profile. Mozbackup (backup) supports it.
- Postbox is based on an old version of Thunderbird, but has added numerous proprietary features. It has more of a Web 2.0 focus with its integration with Facebook, Flickr, Picasa, Gravatar, LinkedIn and Twitter. It has much better OS X support, supporting AppleScript (if you set Postbox as the default email client iCal and iPhoto will use it when sending events and photos), Growl, Lion style scrollbars (you will need a Trackpad) and Trackpad / Mighty Mouse Gestures support on Mac OS X Lion for example. They used to also sell Postbox through the Mac App Store though that has been dropped.
- The layout supports a classic view and a vertical view, but no wide view. The vertical view uses double rows like Outlook does. There aren't any add-ons to add other views such as a Contacts Sidebar.
- Postbox does not have an equivalent of View -> Folders (different types of advanced folder views). Its folder pane is somewhat similar to Thunderbird's Unified Folders with the top half having a unified folder view and the bottom half a list of folders for the currently selected account. However it supports creating custom unified views by creating a new "Account Group", and then dragging and dropping an account from "All Accounts" (or another account group) to it. This makes it easier to separate work and personal accounts in one profile.  
- Postbox does not have a strong user community like Thunderbird does. Version 5 for example only supported 12 add-ons. They don't have any user support forums. According to the postbox blog they are dropping support for all add-ons and the config editor in version 6.0.
- They have stuck with a traditional release process, rather than switching to a rapid release process like Mozilla did. Postbox appears to be using a fork of version 9 of the Gecko layout engine used in both Firefox and Thunderbird, and have no plan to modernize it anytime soon. Having a stable version of Gecko makes it much easier to implement features such as vertical view with double rows like Outlook does. On the other hand it means many years of security fixes have not been added.
- [Active] SeaMonkey is a community project that continued Mozilla Suite when Mozilla stopped developing it as a product and focused on Firefox/Thunderbird instead. SeaMonkey combines a browser with an HTML authoring tool (composer), email, newsgroup, and IRC chat client (Chatzilla).
- The shared MailNews Core code (which includes backend but also UI components like the account manager) is maintained by both Thunderbird and SeaMonkey developers. While the SeaMonkey 1.x Mail & Newsgroups user interface has much in common with Thunderbird 2.0, SeaMonkey 2.0 will share the same backend improvements but likely won't make many visible changes intended in Thunderbird 3.0 (the header pane redesign being an example, moving toolbar action buttons into the message itself). Both applications synchronize their releases and use the same comm-central repository.
SoftMaker Office 2016 for Windows
- [Active] According to the Thunderbird in SoftMaker Office 2016 for Windows article an office suite from Softmaker will stop bundling the "eM client" email client and bundle a version of Thunderbird and Lightning that they have customized. Supposedly they will not create a third party build, instead they will add extensions to provide a import filter for eM client databases and "revert some controversial design decisions made by Mozilla".
- [Discontinued] SpiceBird merges email, contacts, calendaring and instant messaging into one application. It is based on Thunderbird, the Lightning add-on and a communications framework called Telepathy. You can add iGoogle gadgets to Spicebird. Spicebird looked promising but after 4 years it was still only at version 0.8. The project has been discontinued.
Third party builds
- The Third Party/Unofficial Builds forum has several threads for Thunderbird and SeaMonkey builds.
- [Disappeared] Thunderbird -x64 was a third party build of Thunderbird for Windows x64 that appears to have disappeared around the end of 2016. You did not need to use it to run Thunderbird under Windows x64. Since Thunderbird doesn't need the extra memory available to 64 bit applications the main advantage was it runs slightly faster because it is optimized for CPU's that support Intel EM64T or AMD64 and you're not using 32 bit code in a 64 bit environment. See 64 bit builds.
- [Discontinued] Trustedbird is a Milimail project to develop Thunderbird add-ons plus an open-source version of Thunderbird (Trustedbird) that integrates most of those features. Trustedbird 17.0.7 is available for Windows and Linux. It is funded by the French Ministry of Defence and British Telecom. The added features include:
- Address auto completion with several LDAP directories
- Adds deletion receipt to Message Delivery Notification (MDN)
- Enhanced security services for S/MIME (triple wrapping, signed receipt, security labels)
- Limit the message size
- Process the message priority at the envelope level
- XSMTP support (lets you integrate additional headers in SMTP format in the body of a message)