I get and send a lot of email. Many of the emails I send are responses to emails I received. When I respond to email I almost always quote the email to which I'm responding, and when I do my email client (mutt) inserts a line like:
On Thu, Jan 02, 2003 at 11:40:25AM -0600, Justin Chapweske wrote:
Knowing the time of the original message and the time of my reply provides enough information to track my response times to email. I used the inbound message ids to make sure only the first reply to an email was counted.