1. In a server included in an electronic messaging system that receives challenge messages, a method of determining whether a challenge message should be delivered to a first user's inbox comprising: transmitting, by a first messaging service associated with a first domain name, an original message generated by a first user at a first client associated with the first domain name, the original message indicating the first user as sending the original message and indicating a second user to receive the original message, the second user being associated with a second messaging service associated with a second domain name; in response to the original message, at a server of the first messaging service associated with the first domain name, receiving a first challenge message from the second messaging service associated with the second domain name, wherein the first challenge message was generated in response to the first user being unknown to the second user; analyzing, by the server of the first messaging service, the first challenge message to identify whether the first challenge message contains a federated token; if the first challenge message does not contain a federated token, using the server of the first messaging service to deliver the first challenge message to the first user's pending folder; if the first challenge message does contain a federated token, determining, by the server of the first messaging service, at least one of whether the federated token is authentic and whether the federated token is valid; delivering, by the server of the first messaging service, the first challenge message to the first user's inbox upon determining that the federated token is at least one of authentic and valid to allow the first user an opportunity to view and respond to the first challenge message; in response to the first challenge message, at a server of the second messaging service associated with the second domain name, receiving a second challenge message from the first messaging service associated with the first domain name, wherein the second challenge message was generated in response to the server of the second messaging service being unknown to the first user; analyzing, by the server of the second messaging service, the second challenge message to identify whether the second challenge message contains a federated token; if the second challenge message does not contain a federated token, using the server of the second messaging service to deliver the second challenge message to the second user's pending folder; if the second challenge message does contain a federated token, determining, by the server of the second messaging service, at least one of whether the federated token is authentic and whether the federated token is valid; and delivering, by the server of the second messaging service, the second challenge message to the second user's inbox upon determining that the federated token is at least one of authentic and valid to allow the second user an opportunity to view and respond to the second challenge message.