Use another owl.

You approach a perky looking owl perched next to the door. Feeding it a morsel, it willingly jumps onto your arm.

"Can you take this letter to my friend Snuffles?" you mutter to it.

The owl chirps in reply. You tie the letter to its leg, then give it instructions on where to go. The owl bites your finger hard, then swoops into the night. You shiver, and head down to the Great Hall, where there will be a fire and some snacks.