We have an exchange 2007 installed behind Cisco Pix Firewall. The outlook 2003 can send and recieve mails there is not issues but users using outlook express externally and internally cannot send mail. They are getting this error "The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was testdomain.com. Subject 'test', Account: exchange server name , Server: '192.168.100.5', Protocol: SMTP, Server Response: '550 5.7.1 Client does not have permissions to send as this sender', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC78".We can recieve the mails but cannot send. In the incomming and outgoing server setting exchange server ip address is given. POP3 Service and smtp service is running. Anything more configuration is needed for outlook express? Please asist.