We are trying to set manually the RFC822 Message-Id when sending emails using the gmail api. We have tested but seen that Gmail overrides our value. Do you know if there is a reason for this? Can we do something for this?
Yes the Message-Id is always set for mail sending to be the proper format for outgoing Gmail mail, the same format as the web interface.
What exactly is reason for needing to specify your own value instead of using either the message.id value returned during send for your own identifier or the final Message-Id header?
来源：https://stackoverflow.com/questions/25749488/rfc822-message-id-in-new-gmail-api