OLIVIA Alas, Malvolio, this is not my writing, Though, I confess, much like the character But out of question 'tis Maria's hand. (5.1.28)
Malvolio is easily duped into believing Maria's forged letter was penned by Olivia. Here, Olivia's revelation that the letter is a forgery alerts us to the fact that Malvolio is a bad reader. He not only fails to recognize the writing is not Olivia's, he also reads into the letter when he interprets it to be about him, despite the fact that his name is never mentioned.