Gmail Header Format
Because Gmail is a web based application and can be changed at any time, the information in this article may not reflect the current state of Gmail headers. In general Gmail headers have a DomainKeys Identified Mail (DKIM) signature line that contains a signature for the message in question. These lines appear above the standard Message-ID fields. These signatures are of the format:
DKIM-Signature: a=rsa-sha1; c=relaxed/relaxed; d=gmail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:to:subject:mime-version:content-type; b=OITvzFGKQQUjywUQB7U8dQypDAeOGqBIhfcb8VKioP2UU5P2aJL3l2adoyRqSp9h/Fo9A6wY5EIRsfaCWM9ge+EzCob/ 4p85jcEn3uW8dpRyBFQXMuK2q0RMIk3FznrXAM4W5FvoJIPP04qgXErar+/hZq03vEUIErV1v6p2Fy4= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:to:subject:mime-version:content-type; b=oC+hlWhBboQ+RlsKCL4r2pQxpgKRM9iUgCBmw9wZqlEcxj+A3q+fJkDXgLKmI1twfvTHj7GQ3HDzSLzw982UD +CPh1bPQxkhNbylUBRtwpoFeixIk7OmR2YE1iYrYpQXf3dEcXNfKs7ffoeY18plJNJG0S8RRmXLaR6XqXFVUoo=
Note that some of the Received lines will contain hosts with IP addresses like 10.x.x.x. These addresses are non-routable but part of the Gmail system. The remaining headers look like:
Message-ID: <firstname.lastname@example.org> Date: Mon, 5 Mar 2007 09:10:41 -0800 From: UserName <email@example.com> To: OtherUserName <firstname.lastname@example.org> Subject: Subject Line MIME-Version: 1.0
Message Id Field
Although the message id format is not completely understood, it contains the date and time the message was sent, in pacific time, starting at the ninth digit, in the form YYMMDDHHMM. In the example above, the time/date is 0703050910, 0910 PDT on 5 Mar 2007. This has been confirmed by experimental testing. If you are trying trying to determine if a message has been forged, you should verify that the time in the Message Id field matches the time in the Date field for the message. Don't forget that the time in the Date field should be in the Sender's local time and the Message Id will be in Google's local time, US Pacific Time.