The most common problem with e-mail is an incorrect address. It’s easy to mistype an address, or to get the wrong address from someone. If you’re having trouble getting a message to another user, try to verify the e-mail address with the recipient personally.
You can sometimes track down the cause of a bounced message by carefully deciphering the error message that accompanies the returned message. You’ll often find specific reasons that the message was returned, which can help you formulate your reaction.