Paper letters, believe it or not, used to be sent totally unsealed and basically open for anyone to peruse. When privacy became more important, the letters were folded and sealed with wax or another adhesive. When paper became cheaper, it was possible to buy pre-folded envelopes in which to put a letter, and on which the address could be written. (This further enhanced privacy, disconnecting the content of the letter from the recipient.)
While email (electronic mail) is totally digital and cryptographically sealed rather than by wax, the concept of one person sending discrete correspondence to another person underlies the whole system. So it’s intuitive to use the imagery of paper mail; although many who use email may never actually send a letter (or only in special circumstances like voting by mail), they will most likely have received many envelopes.