You can only know whether the recipient has opened the message sent by you. The options are:
1. Read Receipt
2. Embed image in HTML.In order to embed image in HTML, I have done this so that an email is sent from a web site and an image from the website is embedded in the actual email.
...
msg.BodyFormat = MailFormat.Html;
msg.Body = "<IMG SRC=\"cmsm1_multi.gif\">"; //only name of image file
msg.Body += "Any other stuff for body";
...
MailAttachment imgObj = new MailAttachment(Server.MapPath("/") + "web/images/cmsm1_multi.gif"); //actual location of image file
msg.Attachments.Add(imgObj);
... //send the message
Apart from these options, it is impossible for you to know that others have read e-mail sent by you.