Entourage & Microsoft Exchange Authentication Issues

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I have a dicey problem with Entourage version 11.2.4 and Microsoft Exchange. It relates to authentication.

My company uses Microsoft Exchange, and allows 'web based' access to the exchange server. Thus, I have configured Entourage as an IMAP client to my company's server.

Here are the symptoms to my problem. Imagine I have just logged in to my Mac, and invoked Entourage.

1) If I type my username, password and domain into Entourage, the authenticaiton fails. I get an error to the effect that the Exchange server does not reconize my username or password.

2) If I then go to the Exchange web page and log in, and THEN log in with Entourage, it works - I authenticate with Entourage, and I can download, read and write email.

3) If I leave Entourage running, when Entourage tries to access the server again after a period of time, I get the authentication error again. Entourage asks me to retype my password, and when I do this it fails.

4) If I go to the web browser which points to the Exchange web based client, refresh the page, and THEN type my username/password into Entourage, authentication succeeds.

5) Steps 3 and 4 are repeated indefinitely.

I can't believe that I am the only person in the world who has seen these issues. Any idea what is happening here?

Thanks you in advance for your help.