Lindsay Lohan has fired off a letter to "Saturday Night Live" honcho Lorne Michaels -- explaining she was disappointed with him for allowing Miley Cyrus to take digs at her on the show ... TMZ has learned.
Sources close to Lindsay tell us ... she had considered Lorne a mentor and a father figure -- and felt betrayed that he would let Miley mock her on TV -- so LiLo sent an email to let him know she was upset.
In case you missed the show -- Miley finished her opening monologue with a musical number which included the lyric, "I never stole a necklace or got a DUI .. never cheated on wife like that golfer guy ... so what if you can see a little boob from the side ... I'm sorry if I'm not perfect."
We're told Lindsay hasn't received an email back yet.