Email is best so it gets there the fastest. All emails sent to the admissions email address have to be checked because that is where accepted students would send an email letting the adcom know if they are withdrawing their acceptance. They don't have time to reply to every email but they undoubtedly will get it.
But just a word of caution - letters of intent aren't taken very seriously by adcoms because so many people send them promising to attend if they are accepted and then they don't end up matriculating. Also, adcoms know if you have sent a LOI to their school, you've likely sent one to all the other schools you're waiting to hear back from too. This may not be your case, but this is what adcoms have in their minds when they see LOIs. Send one in so they know you're still interested, but its efficacy is questionable.