The “normal” way to set this is: wiki.archlinux.org/index.php/De … plications
For example, assuming your Thunderbird .desktop file is named “thunderbird-bin.desktop”, you could edit “/usr/share/applications/defaults.list” and insert the following contents:
However, as Chromium launches mutt by default, it may indicate you have mutt set as default mailto link opener somewhere. If the method doesn’t work, you could check if you have some settings elsewhere, like, look at what this command prints out and check all the files it displays:
XDG_UTILS_DEBUG_LEVEL=2 xdg-mime query default x-scheme-handler/mailto
And check if you have something in gconf:
gconftool-2 --get /desktop/gnome/url-handlers/mailto/command
If you think xdg-email is used but it somehow is not launching the correct command (at least the xdg-email here (xdg-utils-1.1.0_rc1_p20120916) doesn’t follow the rules), you probably would need to export something like “XDG_CURRENT_DESKTOP=GNOME”. Or you may create a xdg-email-hook.sh if your xdg-email uses it.