Original issue was reported in downstream bugzilla: http://bugs.gentoo.org/show_bug.cgi?id=348409 After updating from XFce-4.6.2 to current version of XFce 4.7.0 (to check the another bug) I've discovered, that in quick-access section of applications menu appeared e-mail client, though this application is not installed on my home PC. I think, it will be good to make this section fully user-configurable. Currently user can only set his favourite terminal emulator, browser, e-mail client. I think, user should be able not only set his favourite application, but configure the list (add/remove application types: for examply many (or even most) desktop users don't need Terminal, I'm thinking about removing File Manager and so long).
No way to check this afaik, you can hide the file with a menu edit tho. So I suppose we need to live with this, unless someone (else) has a good idea.