Undeliverable e-mail

Hi,
I’m an administrator of an exchange using Windows Server 2003 OS and before i can send and received e-mail without any problem (with or without attachment). But now every time i send or any user send e-mail even without attachment i received an auto generated e-mail with subject “undeliverable e-mail” from system administrator with notification ” You do not have permission to send to this recipient. For assistance, contact your system administrator.
“. What should i do? I need help please…

Thank you