The only time I consider that ethical is if the end user knows the email is being tracked. I see it like customer support calls to like HP or Sony where they say "This call may be monitored or recorded". This way people who don't want a record know and can bail on the call. FOr you, I would say put in the fact that you track things in your terms of agreement for purchases or in a contract if they sign one so it's all legit.
"as a fanboy i refuse to admit it and will pull countless things out of my butt to disprove it"
Team Thelegorm! Total Kills: 21 (i iz in uor profile, editsing your sigz)