ENCODING DATA IN EMAIL HEADERS

Among other things, we describe techniques for encoding data that is included in electronic communications. In one aspect, a first electronic communication system sends, to an entity, a first email message that includes a Message-ID field including data that identifies an action to be carried out by a second electronic communication system. The first electronic communication system receives, from the entity, a second email message that includes an In-Reply-To field containing the data that identifies the action to be carried out by the second electronic communication system. The first electronic communication extracts the data from the In-Reply-To field in a message header of the first electronic communication. The second electronic communication system may be the same as the first electronic communication system, or may be an electronic communication system other than the first electronic communication system.