This is a reason to be wary of loading external images by default in mail. I don't know if they do, but spammers could quickly identify active e-mail addresses

This is a reason to be wary of loading external images by default in mail. I don't know if they do, but spammers could quickly identify active e-mail addresses using this technique, so simply opening a mail could mean you would get even more spam, because your e-mail address is now marked as active.