Error setting up Blackberry Internet Service with Outlook Web Access

I'm trying to set up Blackberry Internet Service to connect to our Windows SBS2003 outlook web access. I've tried every possible combination of credentials by I always get the same error:

An error occured during email account validation. Please check your information and try again. If the error persists please contact your System Administrator.

The fields are the following:

Outlook Web Access URL: http://mail.domain.com/exchange (I've also tried just using the IP address http://000.000.000.000/exchange with no effect).

User Name: JohnDoe (same as OWA login / domain username - I've also tried DOMAIN\JohnDoe)

Email Address: john.doe@domain.com

Mailbox Name: This one confused me a little bit, but it seems it should be the same as the domain username (eg. JohnDoe). I've also tried DOMAIN\JohnDoe, and a number of other things.

No matter what I do, I get the same error message. At this point, I'm basically just trying things, because I don't really know how this service is supposed to work. Does anyone know what causes this particularly vague error message, and what I can change either in my email settings or on our exchange server to resolve this?

From serverfault Travis

• The Mailbox Name liekly refers to the Exchange Alias. This often is the same as the user name, but not always. To be precise, if you look at the users properties, the alias is often the same as the Pre 2000 logon name. So check the users account in AD, as the pre 2000 logon name may be different.

  If that doesn't work, check the alias. To do so, follow these instructions.

  From Sam Cogan

• All you want is "mail.domain.com" .. the mail server, not the web access.

  From Trevoke