At least for mailto links, there should not be a second mail client process. For instance, I run claws-mail minimized as a systray icon. If I try to send a file from Thunar with sendto by e-mail context menu, there is called a second instance of claws mail. exo-0.10.2-6.fc20.x86_64 http://git.xfce.org/xfce/exo/tree/exo/exo-execute.c#n146 https://developer.gnome.org/glib/stable/glib-Spawning-Processes.html#g-spawn-async
simplescan uses xdg-email with help from the vala class Process. Maybe worth to look deeper in the module there. http://bazaar.launchpad.net/~simple-scan-team/simple-scan/trunk/view/head:/src/simple-scan.vala#L353
Strange, I also use claws-mail (3.11.1) but I can't reproduce this problem. Using exo 0.10.2-4.
Unable to repro here with claws-mail 3.11.1 and exo-0.10.3.
It sounds like others are unable to reproduce with claws-mail. It's up to the application being launched whether it should run in a single instance or allow multiple instances. Marking this one as invalid. Feel free to reopen if you can find additional debug information.