I have looked over the last few months at a range of email programs - open source, freeware and commercial - some portable - and have come to a few conclusions. I haven't been exhaustive with links to each app, but they can all be found with a few simple Google (or your own favorite search engine) clicks.
Some givens: I will only use email programs that I can easily import/export my data from. All of the email clients discussed below have some import/export functionality. Furthermore, if the email app is not portable - or cannot be made portable - I will not use it. Robustness and speed are my primary concerns. Furthermore, the ability to gracefully handle multiple email accounts, including POP and webmail are important to me. Finally, I want to be able to have the email program display on my PortableApps.com menu. I was happy that these objectives were mostly met.
Some of the more interesting ones are listed below.
1. Thunderbird Portable, the open source email client available on this site, works like a charm - but runs like a dog! It is by far the slowest and most lumbering of the lot. It provides all of the Mozilla magic sauce - regularly updated, quality code, extensions and goodies galore. If only it ran (much, much) faster... I no longer rate Thunderbird highly purely because you can grow old waiting for it to perform its email duties. I have given up using it, and migrated my emails to more nimble packages.
2. Sylpheed - lean email client. Freeware. Inherently portable, but not in PortableApps.com format. Little effort required to make it sing and dance. Does not render html, and the interface is clean and minimal. Stable, fast and full of goodness. I like this one. It was my portable email package of choice for a time.
3. EssentialPIM (Paid and freeware) Available as a portable app. PIM functionality as well as nice email functionality. The paid version allows multiple email accounts. I paid for the pro version, and like it reasonably well. It reminds me of the Linux world's Evolution PIM. Worth keeping in mind. Not at all bad.
4. DreamMail (http://www.softpedia.com/get/PORTABLE-SOFTWARE/Internet/E-mail/Windows-P...) Freeware, and by far the most interesting of them all. Very rich functionality, inherently portable, fast and very robust. This is the killer portable email client. It is not in PortableApps.com format, but is easily worked up into that if you want a different icon, app name, etc... The home website is in Chinese. Some initial concerns about the language issues but far too good to ignore. Yep, it is indeed the killer app.
5. Others? FoxMail (pretty), Claws-Mail (Sylpheed fork and functionally rich), and PegasusMail (quirky but functional). All interesting and good. Claws-Mail is the only one of the whole bunch that couldn't be configured to run (more or less) as a portable easily. It needs the full portable treatment.
In conclusion, I guess that John has made the right choice by offering Thunderbird as the default open source email client on this site - primarily because of its quality and the communities commitment to maintaining and updating it. However, there are some much faster and IMHO much better portable email clients available. The freeware DreamMail is well ahead of any other email app I have tested. Essential PIM and Sylpheed are both good contenders if you want a robust and fast package.
I have no plans to offer launchers for any of the nominated freeware products. But if any of the forum's NSIS hotshots are looking for a new email client project, I believe that you will find a likely candidate amongst the applications mentioned above. Happy coding!