A message relay service receives, from a third-party mass delivery service, an electronic message that includes an electronic address of an entity and obscured data, where the plaintext data of the obscured data is unattainable by third-party mass delivery service. The message relay service identifies, based at least in part on the entity's electronic address and the obscured data, permissions usable to determine if the entity is authorized to transmit the electronic message. If the entity is authorized to transmit the message, the message relay service de-obscures the obscured data to obtain the plaintext data. The message relay service transmits the electronic message including the plaintext data to a recipient specified in the electronic message.