Several emails I've sent are not being received by my recipients. Why?

Several emails over the past couple of weeks that I have sent have not been received by the recipients. Some I've actually received a "mailer-daemon" telling me it wasn't delivered - but not for 2 or 3 days later. The "mailer-daemon" message is not because it's a wrong email address but usually just says that it gave up trying to deliver my message or something to that effect. But other emails, I show the email in my sent-box and do not get a mailer-daemon message and yet the recipients did not receive it. Can someone help?