getRawEmail(callback) {
    const mailparser = new MailParser();
    const { rawEmail } = this.payload;

    if (!this.hasRawEmail()) {
      return callback(null);
    }

    mailparser.on('end', callback);
    mailparser.write(rawEmail);
    mailparser.end();
  }
Parses the raw email and returns the mail object in a callback (Only applies to raw payloads)
@param {Function} callback Function which will receive the parsed email object as the sole argument