There are a few sneaky ways in addition to the per-user URL tracking image people have mentioned. There was some internet drama about facebook (ab)using bgsound

There are a few sneaky ways in addition to the per-user URL tracking image people have mentioned. There was some internet drama about facebook (ab)using bgsound attribute of html mail to detect even when people have disabled image viewing. ReadNotify (as reviewed by jgrahamc[2]) also seems to use similar tricks, including loading remote resources via css url() and similar. [1] http://www.zdnet.com/blog/facebook/facebook-uses-bgsound-to-see-if-you-opened-an-e-mail/10003 http://www.zdnet.com/blog/facebook/facebook-uses-bgsound-to-... [2] http://blog.jgc.org/2006/10/peek-inside-readnotify.html http://blog.jgc.org/2006/10/peek-inside-readnotify.html