Outlook express cannot send mail using exchange 2007 but can recieve mail

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: '', 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.
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Change the outgoing mail port to 587 in Outlook Express.

1. Open Outlook Express
2. Click the Tools menu, then click Accounts.
3. Click your e-mail account, then click the Properties button.
4. Click the Advanced tab.
5. In the Outgoing Mail (SMTP) box, type 587.
6. Click the OK button.
7. Click the Close button.

Hope this helps.

the other option would be that the exchange server wants the user to authenticate.
under the account setup check advanced settings.  there is a section that deals with authentication while sending.
abey_vAuthor Commented:
The problem was solved when i given the authentication setting as annonymous in the default recieve conector in exchange 2007.

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