Some emails - e.g. from commercial sites, merchants, banks etc have html content and sometimes scripts. This is effectively a web page loading inside an email. In such cases, it's possible for you to be tracked or for a malicious sender to detect when a specific mail is opened. For plain text emails, there is no way the sender knows you have opened it unless you respond. Most mail clients and webmail block html by default unless you allow it so unless you enable it they are not going to know you opened the mail. DjKraid's example above is hilarious, but unfortunately people do fall for it.