I can't say for other email hosts, but I use Yahoo. If you go to Sent Items and open the email that you sent, in the bottom right hand corner, just below the bar with buttons labeled "Delete", "Reply", "Forward", and "Move", there's a link labeled "Full Headers". If you click that link, it'll display header data that the server attached when it sent your email -- that includes the server's timestamp -- in my case, I added 3 hours to what it reported because Yahoo's servers are on Pacific Time [Sun, 21 Mar 2010 18:29:28 -0700 (PDT)] and I wanted to report the time as EST.