The closest replacement is to port a copy of vista winmail over to win 7.
To do this is a little envolved,but doable as I had to do this for my wife - she hates change.
Unfornatuely microsoft does NOT like this Idea, they want you to go to the "cloud", so they removed the files from there site. But if you can find someone with vista you can do it. The only hard part is you can not just copy, must first take ownership.
Just google using vista winmail in windows 7.http://www.mydigitallife.info/download-windows-mail-from-vista-for-use-in-windows-7/
Next closest is Outlook which is what I use, but not a "free program"
Other options are as stated above: 3rd party program, or windows livemail.