There is only one email program

There is only one email program for Windows users. No, I haven't lost my mind, and yes Windows users can chose from many client side email programs. But this is a Defensive Computing blog and speaking defensively, that is, with the hope of avoiding problems in the future, there is only one choice when it comes to email programs (webmail is another topic entirely - if you use webmail exclusively you can stop reading here).

Outlook

Outlook is out because it stores all your email in a single file. You don't need to be a techie/nerd to know how dangerous it is to have all your eggs in one basket. A single bad hard disk sector will suck up your time, money and/or email. And because the basket can get very large, backing it up is a pain. Not to mention it's expensive (OK, I did mention it).

Outlook Express

Outlook Express starts with two big advantages, it's free and pre-installed in Windows XP and earlier versions of Windows. And it stores each folder as a separate file, avoiding the big Outlook design flaw. I never liked it, in part because it uses Internet Explorer to display HTML formatted email and thus inherits the security problems of IE. But don't rule it out for this reason alone.
A few days ago, Leo Notenboom wrote that Outlook Express is dead. At his Ask-Leo website someone asked about un-installing and re-installing Outlook Express, a classic tactic for a problematic application. No can do. Quoting Leo: "With the introduction of Internet Explorer 7, Outlook Express has apparently been put out to pasture, at least if you're on Windows XP."
There never was a standalone download of Outlook Express, it was always married to IE5 and IE6. When you updated Internet Explorer, you also updated Outlook Express, like it or not. With the introduction of IE7, Outlook Express was thrown overboard, it's no longer included with the browser.
Thus, if you're currently using Outlook Express on Windows XP, or an earlier version of Windows, you'd better hope it doesn't start acting up. Leo describes a number of ways to try and fix a broken copy of Outlook Express, but none are mainstream operations (I suggest reading the article to see if the fixes are things you're comfortable doing). And his suggested fixes are all Windows things, not Outlook Express things. In my opinion, you're better off using an email program that is not an integral part of the operating system.

Windows Mail

Windows Mail is the replacement for Outlook Express in Vista (it only runs in Vista). According to Leo, there is no stand-alone download of Windows Mail, so it too can't be easily un-installed and re-installed and is, perhaps, too much a part of the operating system. Also, it's new and thus likely to be buggy.

Windows Live Mail

Leo Notenboom updated his posting September 1st to include Windows Live Mail, an email program that neither he nor I was aware of. It's a new version of Outlook Express that runs on both Vista and XP with Service Pack 2.

First off, I can't believe the name. Microsoft learned nothing from the confusion they caused non-techies by similarly naming two totally different email programs (Outlook and Outlook Express). My guess is that it will eventually be referred to as Live Mail, both because the "Windows" is superfluous and to help differentiate it from the Vista-only program (which they should have called Vista Mail).

Whatever it's name, the software is in beta, so the jury is still out. Except, that is, when choosing defensively. Beta software is out of the question when it comes to applications that really matter to you.

Thunderbird

I recommend Thunderbird from Mozilla, the same organization behind Firefox. According to Leo Notenboom "Thunderbird is free, fairly similar to OE to use, and actually somewhat more powerful. It's free, downloadable, it's being updated, works on Windows XP and Vista as well as the Mac and Linux, and there are many add-ons available for it."
To this I'll add that Thunderbird, like Firefox, is very good about updating itself with bug fixes. Keeping your applications up to date is a great defense against malicious software. And since Thunderbird does not use Internet Explorer under the covers to display HTML formatted email, it's safer still.
The safety provided by Thunderbird comes at virtually no cost. Not only is the software free, but it's easy to use. I say that not based on my own use of the program but based on the reaction of many of my non-techie clients.
You can download Thunderbird from Mozilla or from download.com where the Editor's review gave it 5 stars (out of 5) and where 511 users (as of September 1, 2007) rated it 4.5 stars.

Eudora

Eudora is liked by many techies but it's in transition and thus I'd be wary of trusting it with my email. The official website says "The Paid mode commercial versions of Eudora are no longer available as of May 1st, 2007. The Sponsored mode versions of Eudora continue to be available for download. An open source version of Eudora® is being developed by Mozilla and will be free of charge."
To translate, "sponsored mode" refers to a free ad-supported version. While free is good, abandoned is not. The new open source version of Eudora is called Penelope and the first beta was released August 31, 2007. Any brand new software is likely to be buggy for a while. I'll pass.

Lotus Notes

Perhaps the most hated email program to ever walk the face of the earth.

Updated September 1, 2007: Added Lotus Notes, Windows Live Mail, link to download.com for Thunderbird and Penelope.