Deadlines

Forgive me if this is a silly question…when there are job listings that ask for a hard copy of resumes and cover letters to be mailed by a certain date, do they have to physically be in the office by then or is that just the postmark deadline? Will they read it if it gets there after the listed date? Trying to figure out if I still have time to send in my info to Cosmo for the fall internship—due tomorrow :-/