(3.11.1 - Latest version in Ubuntu repository)
Click on "mailto:" link prepends http:// to link and attempts to open in browser. Workaround is to switch to plain text view (instead of Fancy) to click on link - works as expected.
I cannot reproduce that. I guess it's a problem with your email. Can you attach it here or privately?
Fancy passes all links (including mailto: links) to an external browser. Non-Fancy (plain text) view automatically displays email addresses as links that it processes itself. This explains why part of claws worked and the other part didn't. The problem was with the browser, not claws.