This page summarizes a lot of data about when people open email, and a lot of other topics. The most interesting and important to me, is the table (near the bottom) that lists which email client (including most of the popular stand alone and web based ones) handle graphics and whitelists.
http://www.emaillabs.com/resources_statistics.html
Slashdot on Outlook 2007, plus some interesting email marketing links in the comments:
http://developers.slashdot.org/article.pl?sid=07/01/13/1911226
Email headers: "In general, the Sender is the actual originator of the email message. The From Address, in contrast, is simply a header line in the email that may or may not be taken to mean anything. The From Address can often be left out completely. Spammers can easily spoof the From Address. ISPs try to ensure that spammers cannot spoof the Sender."
http://www.tectite.com/fmdoc/from_vs_sender.php