You're going to get a mixed bag of personal preferences here, and for varied likes and dislikes that don't necessarily equate to visual ease of use.
It's like comparing (1) some of the image viewing and editing programs that are installed from CDs that come with scanners and printers with something like (2) the lower end Adobe image editing products. All the same features may well be there lurking underneath the "skin", but:
1. The first gives the user large rounded "buttons" to clumsily click on while fancy sound effects confirm that the user clicked accurately
OR
2. The 2nd more professionally laid out offerings feature complicated looking floating toolbars and a plethora of rarely used menu options available from standard menus or right-click options.
The problem with the first type of interface is that, despite trying to make the functions intuitive, these over-simplified interfaces often make you hunt for the lesser used options (the button images are often ambiguous), or make you skip back and forward without visible means of getting back to where you were before you had to click on a button to see what it did.
The problem with the 2nd type is that, although the menus and toolbar buttons are clearly labelled, new users are often overwhelmed by those visible options and get the immediate impression that there is too much to learn.
In actual fact, the oversimplified interfaces and the more professional ones require about the same amount of learing through repetition. So what it comes down to then is whether the user needs to SEE ALL the functions available, or will ever use them.
Anything that allows a user to "easily" read and send rich text emails with attachments and embedded images, etc, while still being protected by an antivirus program and having a spell checker, is either going to be a skin over one of the popular free email clients, or will be a lesser known open source or freeware offering with quite a few bugs. By "easily" I refer to what you are asking, ie. something easy to use for seniors and something YOU will find easy to support.
From a personal viewpoint, Mozilla Thunderbird is very customizable using any of the huge number of skins written by and for all kinds of people. It offers all the functionality needed, can be made to LOOK simpler in operation by you, and there are plenty of support resources for you to troubleshoot problems.
I liked Opera when I last used it, and it appears to have progressed with the "Vista" feel, judging by the screenshots. I think it would be worth a try out. It allows migration of contacts, etc, from a number of other clients.
Windows Mail (Vista) and Windows Live Mail (XP add-on) are chunky and attractive in appearance, but were designed to emulate Outlook Express and provide a half-way point between that and Webmail. Apart from loading in a bloated slow way, I find that it is too much like Webmail in the feel of the Windows. It also breaks down too easily.
In the end, no matter what interface or client is used, it has to be shown in such a way that messages are listed and provide the means to click or double-click to see them fully unless a "preview" is provided. That, in my opinion, makes Outlook Express and Windows Live Mail no harder or easier to use than webmail.
http://www.techsupportalert.com/best-free-email-client.htm
