Windows Mail (NOT Windows LIVE Mail) continues to run perfectly now on four Windows 7 PC's that I personally know of & if anyone's running the 64-bit version, it'll run on that too (but requires a different dll). IMHO, way better than any other presently available E-Mail client including those mentioned above. It's a pity Microsoft are on off in another direction & wanting to encourage people to use their "Live Mail" which is the "pits" as far as I & my friends are concerned. M/S’s Outlook which is also good, is far to complicated for many even if they have Office installed. Incredimail is for the ladies & anyone else who likes frills & fancies all over the place. I don’t. Hardly suited for business use at all.
Outlook isn't all that complicated... and it does have a lot of features that the free MS clients don't. It's much better to find a viable alternative that is fully compatible with Windows 7; as you've already found out, MS can break compatibility at any time and they will feel no obligation to fix it.