Most mass emailers (Constant Contact, Mail Chimp, etc) include a 1x1 transparent gif in their emails to track “opens” as a feature. The senders can then see who has opened the email they sent.
The code would be like…
<img width="1" height="1" src="http://mailservice.com/viewtracker.php?userid=12&newsletterID=70202&email=someone@domain.com">
Opening up an email with that code in it would hit a php page called viewtracker and the sender’s user ID, the newsletter’s ID and the email address of the recipient would be tracked.
You can thwart the tracking by not allowing images to display in your email program, whether it be Outlook, Thunderbird, GMail, Hotmail, etc.
But, it’s not uncommon for a tracking image to be in an email newsletter. In fact, most mass emailers would be useless if they didn’t do this, as the whole point of sending out newsletters is to see who’s opening them.
Unfortunately, while it’s a very good thing for legitimate companies wanting to do successful email campaigns, the same technology can be used by spammers to verify that addresses exist. That’s why using the Preview Pane on your email application can be a bad thing, if you don’t turn off images and you view spam before deleting it.
The fact that evite uses this technology is not nefarious on the surface, but if they cull the addresses and sell them then it is a bad, bad practice. Then again, they are undoubtedly storing your email address when someone sends you an evite whether you open the email or not, or reply or not, so they’ve got your info anyway. Collecting information on whether or not you viewed and whether or not you responded is actually good information to supply to the account holder…but once again, bad news if they are sending it to third parties.
Whether or not they do send to third parties is something we may never know, but that might be how a free service like evite makes its money. (not to say that this is a very nice way to make money, or that I approve of it)