Each student has a personal SU Box where they receive campus and postal mail. SU stands for Student Union, from the time when the boxes were located in the student union (Baxter). They were relocated to the four dining halls during the construction of the new student center and a building number was added to every student's address (Mission Park- Building A, Greylock- Building B, Driscoll- Building C, Dodd House House- Building D). They are now in the new Paresky Center.
The location of boxes during the Paresky Center construction depended on your geographical location on campus but, contrary to popular belief, did not depend on the cluster you are in. For example, the residents of Spencer, Mark Hopkins, and Bryant all had their mailboxes in Greylock, which would lead one to believe that all members of Spencer neighborhood had their mailboxes there. However, this is not true, as the residents of West had to trek all the way to Dodd to get their mail.
Combination
To open your box you need a combination which you receive at the beginning of the school year, mailed to your home. If you lose or forget that combination, you can always go to the campus post office and ask about it. It is absolutely free.
Opening your mailbox is a little tricky. You have to turn right three times before stopping at the first number of the combination, then go left passing by your second number once and stopping at it the second time, and then go right again to the third number. Now, push the knob in and turn it to the left and pull the box open.