If you have tried to send e-mail to a valid e-mail address and the mail comes back as being rejected, the most common causes are:
Specified address is not valid - Check your spelling. Check for extra characters on the end of the address, such as a trailing period. The last character of the address MUST ALWAYS be a letter or number and is never punctuation. (Mail to "email@example.com." will never be delivered because of the trailing period.)
Specified address is no longer valid - Did the correspondent move? Or is it one of those addresses that only work for a limited time?
Specified mailbox is full or disabled - You need to wait or contact the mailbox holder by other means and alert them of the problem.
Mail was rejected because of its size or content - Try sending something much smaller with only text and no attachments and see if that works.
Specified mail system has been broken for an extended period of time and was unable to accept the mail in an allotted amount of time. - Try contacting the mailbox holder by other means and tell the person about the problem, or wait and try again in a day or so.