E-mail. (Hence the smileys.)

Frank was asking about the meaning of “RSVP” in the invitation letter, so I made a ‘dict’ search for it. Here go the answer :)

2 definitions found
From Virtual Entity of Relevant Acronyms (Version 1.9, June 2002) [vera]:
Resource reSerVation Protocol (IP)

From The Free On-line Dictionary of Computing (27 SEP 03) [foldoc]:
1. {Repondez s’il vous plait}.
2. {Resource Reservation Protocol}.
3. {Revolutionary Surrealist Vandal Party}.

Will it be the Surrealist Vandal Party one? :)