Yeah I have (had) the same hassle on OpenSuSE 11 using KDE4. I tried everything. Eventually I blew the install away (including web server, steam server, ftp etc...

) The wife uses this rig as her interweb machine, and sadly... a hotmail devotee. I installed Linux Mint to no evail, Slackware, Ubuntu and RHEL all failed to compose and email,and yet my EeePC with the Xandros install has no hassles at all??? Couldnot compose under seamonkey, konquerer or firefox... This is a coding issue...coders???
I have migrated the wifes email and contacts to our hosted provider and set up a forwarding rule in hotmail.
My advice is to leave the Redmond, California USA mail client as quickly as your little fingers can type. I for one am glad for the f&^k up, as the wife no longer uses the stupid thing. I am however dissapointed that much of my hard learned efforts got blown away due to this little hiccup.

More due to the wifes complaints than MS's changes.
As a MSCT I am very dissapointed. As a MSCT I am not surprised. With a scheduled shift towards INTERNET BASED APPLICATION SERVING, this kind of lock out was inevitable. My expectation is that eventually MS will have its own "global domain" or similar. The sooner they get that concept up and running means they will maintain their currently shrinking audience. A captured audience. If ever there was a real threat to the growing *nix comunity, check out the reported abilities of Windows 7.
Waht does the sendmail project have to say?