Delivery tickets that include data and a checksum are used to authenticate incoming electronic messages on behalf of a user. The delivery ticket is located in a field in the envelope portion or in a header in the content portion of outgoing electronic messages. A bounce message or a reply message generated by a remote computer in response to the outgoing electronic message includes the delivery ticket. When the bounce message or the reply message is received by an authentication server associated with the user, the delivery ticket is authenticated to determine whether to deliver the incoming message to the user. The delivery ticket is initially validated if a checksum regenerated by applying a private key to the data of the delivery ticket is the same as the checksum included in the delivery ticket. The validation process also includes determining whether the delivery ticket complies with rules that specify the duration of time of the validity or the number of times that the delivery ticket can be used.