Exchange 2010 and possibly 2013, by default interestingly allow only 10 ‘ActiveSyncDevices’ (defined device-connections, ActiveSync partnerships) from things like smartphones and tablets. In theory it should be possible to increase the limit according to e.g. http://www.msexchange.org/kbase/ExchangeServerTips/ExchangeServer2010/Mobility/HowtoincreasethemaximumnumberofActiveSyncDevicesforaUser.html . This for me however didn’t work.

Microsoft considers this a client side affair, to be managed through their OWA, Outlook Web interface by the user, not the administrator (#*$&#%#!). There is however the possibility to at least get rid of unused ActiveSyncDevices through the Exchange Management Shell. With thanks to http://thoughtsofanidlemind.com/2014/01/15/eas-device-partnerships/ and http://www.itexperience.net/2014/07/18/activesyncdevice-found-exchange-2010-removing/ , the only way that really worked for me is by using Remove-ActiveSyncDevice -Identity with the GUID as the argument. This can by found by

Get-ActiveSyncDeviceStatistics -Mailbox <mailbox name or alias> | Format-Table GUID, LastSuccessSync

The LastSuccessSync gives a clue about whether the device is probably still in use or not.