Call the person you are emailing to find out if her or his mailbox is full (that is, over the permitted size limit), disabled, or not working for some other reason. Email servers will reject mail sent to full or nonactive mailboxes.
Check the email error report for explanations of the delivery failure. The messages in this report provide important reasons why your email couldn't be delivered.
If other people can receive email from you, wait a while and try sending the "undeliverable" email again. Often problems like this are intermittent, and waiting a little while is all that it takes.