My Claws Mail got a little buggy, so I installed the lastest Sylpheed in order to check it out. I wanted to use Claws as my main email client, and keep Sylpheed installed as a backup. However, I could not change the default email client back to Claws, not thru Claws nor thru Windows. And when I try to change it thru Claws (in the Help menu), Claws hangs, even after I uninstall Sylpheed
Perhaps Sylpheed mangles the related Windows Registry entries. Have you tried reinstalling Claws Mail? The entries are set during installation only - I don't think Claws Mail itself checks them anytime later.