So apart from senders address on the envelope, the letter included another header stating the location and the date. Where and when the letter had been written is part of the letter. Another clue as to from where the letter is sent is the postmark on the stamp.
The way I understand email headers is that they are for adding metadata to the content of the email, similiar to the envelope. First and foremost of course, it includes the addressee, but also the sender's name and email address. Another header is the date the email was sent at. In the old days, an email address may have given you a clue as to where the person is located, the first email to arrive in germany was sent to "rotert@germany", which was a bit unspecific, but you get the idea.
Things have changed and now most people have an email address that doesn't give a clue about their current location. Sometimes other email headers may provide a clue, for example IP-addresses can be geolocated, but since the rise of smart phones, emails are written from anywhere. The simplest way to let the recipient know about your whereabouts is by simply writing it into the body of the mail, but that's not really machine readable.
I propose introducing an optional email header that specifies the senders geographic location.
X-Sender-Geo-Location-Placename: Berlin, Germany
If a location is provided, accuracy is required, the address field can be used for displaying, if not available it can be reverse geolocated. But those are just details. I think parts of the geolocation API can be used or adopted, like altitude, heading and speed.
I would love to hear what people think of adding a geo tag to email headers. It shouldn't be too difficult to write thunderbird add-on that let's you specify these header components and display a map if they are present. Or a gadget for gmail.