According to the Wiki page on '
Hugs and kisses' (attribute to this source as much deference as you wish), "The common custom of placing "X" on envelopes, notes and at the bottom of letters to mean kisses dates back to the Middle Ages, when a Christian cross was drawn on documents or letters to mean sincerity, faith, and honesty. A kiss was then placed upon the cross, by the signer as a display of their sworn oath."