even then theres ways for the reader to get round it
for instance, if like me you use "The Bat!" or "Zimbra" as an e-mail client then those 1-pixel web tags will never get displayed unless the default program begaviour is changed, Its one of the reasons I use them. I also have sending of read receipt replies disabled......
Even in Outllook Express the recipient may have specified "text only"
and of course you can block cookies as well - though its less likely the recipient has done that