I wonder if there are still people who write letters with a pen? If so, do they draw smiles in the hand-written text? Personally I haven't received anything but cheques for taxes in the last 15 years. Post mail is slowly dying out.
However, a lot of emojis represent the process of sending and receiving letters, both electronic and paper. One of the most popular icons is an envelope. You can see it on various websites pinned in the contact section right before an email address.