It seems like all of us have a friend or three that doesn't follow proper password security, gets hacked and you receive spam from that friend's account. If you mark that email as spam you have the option to instead mark it as suspicious -- this keeps you safe from future spam from your friend's account but will allow regular email through.
This tip comes from former Lifehacker Editor-in-Chief Adam Pash, who discovered the feature while receiving spammy email from friends. If you mark the email as suspicious it will send data that is designed to help prevent account hacking but won't penalise your friend's email importance for future messages and also encourages you to contact your friend to let them know their account is hacked. If the email is from an old email address that is no longer used you can choose the "Message is just spam" button and the email address will be removed from your contacts list.