Message-ID, is a unique identifier assigned to individual email messages by the email server responsible for sending the message. SendGrid will create a Message-ID for each individual message you send through our systems. This Message-ID information can be viewed in certain events in the SendGrid Activity Feed as well as Event Webhook POST data. The Activity Feed references the Message-ID as “msgid” and the Event Webhook references it with “sg_message_id”.
Example SendGrid Message-ID: Message-ID: XBg2anf2TqCy6WXKQFhieQ.filter0905p1mdw1-4434-59E0C6FF-3.0