iOS 4: Microsoft Exchange Server 2010 SP1 accepts only ten devices per account

Microsoft Exchange ActiveSync 14.1, included with Exchange Server 2010 SP1, introduces a default limit of ten devices per Exchange account. If you configure an iOS device that exceeds this per-account limit, you may see unexpected behavior: Mail and Calendar accounts become unresponsive after creating a new Exchange account, or after the server for an existing Exchange account is updated to SP1. When installing a profile that configures an Exchange account, the message "An unrecoverable error occurred while validating the account information" appears, and the profile fails to install. When you check email on another device, you see the message "Error with your new mobile phone partnership," which says "You have 11 phone partnerships out of the maximum allowed 10 partnerships."

Source: Apple - Support - Most Recent - iPhone