http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message

Thank you for the info, but I already sent the emails, so it doesn't look like I can retroactively do those suggestions. Is there a way to see if the email went through that was already sent without doing return receipts or DNS?

Thanks!

Typically you'd get a delivery failure notification in case something went wrong. Even if there's no error message, there is no guarantee that your message has been delivered to the recipients Inbox. E.g. the message could have been classified as junk by the recipients email provider, or the recipients email client.