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18 May2015

LEX Technical Bulletin:
Exchange Server & Exchange 365

David Taylor

LEX utilises Exchange Web Services (EWS) in order to update diary appointments to the Outlook Calendar and has done so since Exchange 2007.

Access to mailboxes can be achieved via the EWS API in two ways – delegation and impersonation.

Delegation is achieved on behalf of one administrator mailbox being updated and requires full read/write and send permission for the box.

All operations are added to a single “cost” total. This was the method adopted when LEXeX diary synchronisation was developed for Exchange 2007.

Impersonation (Exchange 2013 onwards) does not require this level of permission and is inherently more secure as the application impersonates each user and inherits their level of permissions to update each user mailbox.

Every update to a mailbox has a “cost” that adds up over time on the affected mailbox to assist the throttling policies implemented by Microsoft. These policies ensure that Exchange Server can service all accounts in a timely fashion and throttle large running tasks so as to effectively stop denial of service attacks and maintain an acceptable level of server performance.

Operation “costs” are assigned on a per mailbox basis (one single total for delegation and individual mailbox totals for impersonation). Over time the “cost” of each operation on a mailbox will reach a pre-determined limit, at which point the synchronisation process will fail as Exchange Server denies access.

Delegation will reach the “costs” limit within a number of days as it uses only one administrator mailbox, whereas impersonation will be protected from reaching throttling limits as each mailbox has its own limit.

It is therefore essential that LEXeX utilises impersonation rights in order to protect itself from denial of service attacks and we have no choice but to adopt this method of interacting with mailboxes from 2013 onwards.

As LEX chambers may currently use either delegation or impersonation, depending on versions of LEX and/or Exchange server, it is vital that Exchange Server migrations are not undertaken without prior consultation with our Helpdesk.

A number of additional processes are required to clear out existing synchronised items and replace them with newly formatted entries that are capable of operating successfully with the new versions of Exchange Server and Exchange 365. Charges are applicable for this service.

Without consultation in advance LEXeX diary synchronization will cease to function and Outlook calendars will not reflect correct court appointments.

Please circulate to all personnel responsible for Exchange support and migration on your behalf.

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