Molon, I think you have the option to sell to whoever you want for whatever reason you want. If you want to use a time stamp then that's a problem. If someone comes forward and says they sent an email before me I'll have no problems with you selling it to them.
It's a bummer the email didn't work but I know as a seller too that sometimes PM's don't work either. It seems to me the only real time stamp that you/I can rely on is the reply.
FWIW, here's the time stamp on my email (since I had a copy sent to myself):
Sent: Mon 2/26/2007 12:24 AM
So, it turns out it was just after midnight Mountain time. Of course, I could have edited it but I didn't.