code: function toInboundMessage(relayMessage, mail) {
      let inboundMessage = transport.defaultInboundMessage({
        to: {
          email: relayMessage.rcpt_to,
          name: ''
        },
        from: mail.from[0],
        subject: mail.subject,
        text: mail.text,
        html: mail.html,
        recipients: mail.to,
        cc: mail.cc,
        bcc: mail.bcc,
        headers: mail.headers,
        attachments: mail.attachments,
        _raw: relayMessage
      });
  
      inboundMessage.id = transport.defaultMessageId(inboundMessage);
  
      return inboundMessage;
    }
docs: Takes a relay message from sparkpost and converts it to an inboundMessage
  
  @param {Object} relayMessage
  @param {Object} mail - from MailParser
  @returns {InboundMessage} inboundMessage