The most important feature of my email client is that it allows me to deal effectively with incoming mail for many addresses. I would love to have just one email address to cover all facets of my online existence, but I don't. I have lots. Let's say I own and run 3 domains - a.com, b.com and c.com. At the moment all mail to these domains is forwarded to my Mac.com account and picked up using the Mail.app client, or online. The problem is that Mail.app assumes one identity when replying to all messages. If you mail me at any of the domains (i.e email@example.com) and I reply you will then see a different transient address (i.e. firstname.lastname@example.org). Not only is this confusing for the receiver ("Which address should I mail you at Jake?") it looks unprofessional if the mail were to a business domain. It also means lost mail if I drop this transient account in the future and people still think that address is your most recent.
What I really really want my perfect mail client to do is use some common sense. If I reply to a message I want it to appear that the reply was sent from the same address as it was sent to. If a message arrives for email@example.com and I reply it has got to look like I am sending the reply as firstname.lastname@example.org. The same applies to all addresses at all domain. I've searched in vain for a mail client that has the option "Set From address in replied equal to original message's SendTo address".
I would be happy if this were the only criteria met but I also want the system to store all my mail in one place. This would preferably be on a server as I would also like to login from wherever I may be. Hence the online client also needs the above addressing feature. Least important but still a factor is the look of the client. It must look good. Mail.app fits the bill but it has the usability issues of the Mac. I want a nice-looking, lightweight program to run on Windows and help me manage my email madness. Bayesian spam filtereing would be nice as well.