Absolutely pathetic. A post-it note, a quiet word, or any of the harmless pranks suggested in this thread (
@Broken_Erika's suggestions have gone sadly unnoticed IMO) would do the same, without the immaturity, illegality, idiocy and hypocrisy of your "prank". Grow up.
The fact that "I should be allowed to send unauthorised sexually suggestive emails from another person's professional email account and be applauded for my commitment to computer security" is the hill you're willing to die on speaks volumes.