getting exception in sending mail

hello everybody,
i am using database mail for the first time.
i have configured it by using both wizard and stored procedures.
but i am getting the error in the log file as:

"The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 4 (2007-09-04T12:32:55). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 )"

Please help me out.
Thanks in advance

Here you go:

http://msdn2.microsoft.com/en-us/library/ms187540.aspx

|||hi Buck,
thanks it helped me a lot.
regards,
jasmine.