That's why email programs now default to disabling linked images and only displaying embedded images by default. The spammers were containing a 1x1 pixel transparent image on their site that they linked into the emails that they sent and insecure email programs would then add an entry to the spammers log indicating that some fool had actually opened their spam.
Any indications that you do get that email wasn't delivered indicates that the recipient's mail server is insecure and broadcasting information to spammers as to what email addresses exist on that server. In that case the most likely reason that your email cannot be delivered is because the recipient's mailbox is full due to all the spam sent to them.
Where their email server and email program are properly secured to prevent spammers from knowing whether their email address exists or not then any emails you send them that cannot be delivered will not tell you that they didn't get there.