Is it possible to add headers to emails being sent by app-engine?

I'm currently putting the finishing touches to an application for a client, this application sends daily emails to subscribers, is it possible to add custom headers to the email so that certain variables can be tracked, like day number (X-开发者_JS百科Day-Number) etc...

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There isn't a documented way to do it. Perhaps you should suggest it on the issue tracker. http://code.google.com/p/googleappengine/issues/list

Of course, if you're really determined, you can create your own email sending gateway on a more traditional server, and call its web service from App Engine.

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from looking in appengine/api/mail.py

mime_message = myMessage.to_mime_message()
mime_message['message_id'] = 'myid'

Thanks to https://stackoverflow.com/users/396269/derpderp