Just so that I can agree and disagree with j2buttonsw and xyzzyplugh I think the answer is Yes/No and Maybe!!
If the recipient replies or acts on your email the answer is yes. If you get an email which says the recipient address is invalid the answer is no. Otherwise you can not be certain because of several factors. The recipient could have logged onto their ISP and deleted the mail straight off the server without opening it. The other possibilites depend on whether you requested a receipt and how the recipient has their mail client set up. I will use Outlook Express 6 cause that is the client I use. If their mail client is set up to never send a receipt, you are not going to get one even if you request it, or if their client is set up to Always send a receipt you will get one whether you requested one or not, the other choice is that the client is set up to notify the user if there is a read receipt request as part of the email. So if you request a receipt the recipient has the choice of clickng Yes or No to the question of sending a receipt. In Outlook Express the same three options (Never/Always/Notify) apply to secure receipts but you can set the values differently for secure and unsecure receipts. I assume other mail clients act pretty much the same way.