If you are having trouble sending a message to someone:
Check the email address of the person that you're sending the message to. Make sure that you include the entire address (for example, “email@example.com”). If you don't include the domain (such as “@example.com”), the mail server can’t deliver your message.
Make sure that the email address is current. That email account may be closed, or the recipient may be using a new email address.
Ask the recipient to make sure that your email address isn't on a blocked senders list, and that the message wasn't delivered to the Junk E-mail folder.
If you receive a return or "undeliverable" message, read the information carefully. It often explains why the message wasn't delivered successfully. For example, there may have been a problem with the recipient's mail server.