The nature of the invitation determines the correct method in which to RSVP. For example, for an invitation received via mail that includes a contact number, an RSVP should be given through a phone call, preferably in person and not on an answering machine or through voicemail.
An invitation that states that text messages are welcome may be responded to with a text message. For invitations sent via email, it is acceptable to RSVP by replying to the email invitation, according to Emily Post. For an Evite, the included directions should communicate how to RSVP. If an invitation is received over the phone, it is acceptable to say that a personal calendar must be checked before committing to an invitation; however, it is necessary to phone the host back to let him know of a final decision.
Some invitations contain response cards, and if this is the case, the card should be used to RSVP. When invitations come via mail without response cards, it is expected for the RSVP to come as a mailed, written response. Invitations that state "Regrets only" require contacting the host only if the invitee is unable to attend. Wedding invitations that do not contain response cards should be responded to either formally or with a personal note if the hosts are well known by the invitee, according to Emily Post.