I think that another question might be "How can I get people to put in real email addresses and not disposable ones?" But, in answer to your question: If you s

I think that another question might be "How can I get people to put in real email addresses and not disposable ones?" But, in answer to your question: If you send a few (legitimate - not spam) emails to a recipient, and they all bounce, then that's probably a disposable one that expired.