Let me provide you technical but user-friendly answer.

• MS Exchange server recognizes 2 types of devices - provisionable and non-provisionable.
• Provisionable devices can be "provisioned" by MS Exchange server with different security settings. E.g., "Autolock feature should be activated", "Autoloc period should not be greater than 10 minutes", "The length of the unlock code should not be less than 5 symbols" etc.
• Non-provisionable devices are considered to be dummy - they can not handle these requests.

Server administrator has check box "Allow non-provisionable devices". If this box is turned on, server works with such devices. If it is unchecked, it rejects all the connections from them. The value of this option depends on the company policy. According to my personal observations, companies with US HQ tend to have this option disabled while the companies with EU HQ tend to have this option enabled.

N900 was non-provisionable device. N9 can be provisionable or non-provisionable device depending on the value of the UI setting in question. Obviously, you can't change the value of this option after setting the account up.

What is the benefit of turning this UI setting? E.g., your server security policies are stupid from your PoV (something like, autolock should be activated in 5 mins inactivity time) but in the same time non-provisionable devices are allowed by server administrator.