Exchange 2007 POP3 and IMAP problem

Recently I have installed Exchange 2007 at customer site, and working fine. Now customer want to use POP3 account for some outside users.

I have just enabled POP3 service and configured the account for outside user in outlook 2003/2007. They can send and receive e-mail in their company but unable to send outside world like yahoo, gmail...while they are able to receive the e-mail from anywhere like yahoo, hotmail etc

When they send message, getting error below (Only with POP3 account):

Your message did not reach some or all of the intended recipients.

      Subject:      RE: test mail
      Sent:      11/18/2008 8:46 AM

The following recipient(s) cannot be reached:

      '' on 11/18/2008 8:46 AM
            550 5.7.1 Unable to relay

      '' on 11/18/2008 8:46 AM
            550 5.7.1 Unable to relay

This means we can only receive the e-mail in pop3 OR IMAP. when I talked on Expert Exchange, got suggestion can be receive e-mail but can not send outside world like gmail, hotmail.

Can anybody clarify me how can i get the benefit of pop3 account in Exchange server 2007 with Send-receive.

Note: Outlook anywhere can not be configured for outside users bcse Certificate is not installed and also right now customer does not want to purchase certificate .

Kindly Suggest me............Thanx in advance..
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This is the Problem of SMTP Relay. You SMTP is not allowing the annonymous Users to send the Message.
Just in the Users Outlook Profile Click Modify the SMTP Settings to use user login infromation.

mahtab786_dbiAuthor Commented:
Same problem

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