Might be easier if it's simple, but maybe a search bar like Gmail might be easier to predict, and give the user the option of advanced filters.
There's no clear idea of what that icon is supposed to mean for me(icon_mail_sp). It might be the ones that have been sent? And how difer from those who hasn't been sent?
I would've put it on the right or on the bottom of the page, because there's a lot of information in the left right corner of the screen everytime, even on mobile.