It's written somewhere - though whether on the front or inside the letter isn't quite clear. The letter doesn't come in an envelope, but folded and sealed with wax (Malvolio refers to the "impressure" and the "wax"). Malvolio reads the following lines on the front, unsealed part of the letter:
To the unknown beloved, this, and my good wishes.
After that, the love poem which you refer to could be either inside the letter or also written on the front: Shakespeare doesn't provide a stage direction "He opens the letter" unfortunately.
Jove knows I love,
Lips, do not move,
No man must know.
I may command where I adore;
But silence, like a Lucrece knife,
With bloodless stroke my heart doth gore;(100)
M, O, A, I, doth sway my life.
And after that, the love poem ends and it becomes a love letter: as Malvolio says, Maria's letter moves from rhyming verse (and Malvolio refers to the "numbers" - the rhyme scheme) to "prose".