I am sending emails using Mandrill and I want to set the Message-ID myself. I see now an ID like this:
Message-Id: <30152892.20150311183809.55008b919f0c49.696
Yes, you can set your own, but you will want to be sure that it follows the appropriate format for a message id. We won't override your id as long as it appears to be valid, but if it's not formatted correctly, we'll overwrite it.