This is what Slade Smith sent me last year, and I just did it again to solve the same problem. If you use Windows Live.... be sure to also open that unit and check the defaults.

"Nils, this is on your end; the envelope is just a link to an email address. Your browser determines which program is opened when an email address link is clicked. In IE this is changed by going to Tools –> Internet Options –> Programs –> Set programs, clicking the desired program to use for email, and then checking the checkboxes for all the desired file types. In other browsers, you’ll do something similar".

I just did it again in IE and Chrome and we are back in the saddle again!