Hello,

Is there any way to get an immutable id for a given mailbox via EWS?

As I know user email address can be changed so it cannot be used for identification of the mailbox.

What is about UserDN and MailboxDN? is any of them immutable?

 With EWS the PrimarySMTPAddress of a Mailbox is the Key you need to use, I guess the question is why do you need an immutable (there is one for Directory sync between Office365 and OnPrem) . In Exchange the immutable Id for a Mailbox is the Mailbox GUID https://technet.microsoft.com/en-us/library/aa996437%28v=exchg.65%29.aspx but you can't get this value via EWS . The MailboxDN (or LegacyExchangeDN) is the X500 address of the Mailbox which shouldn't change although it is changeable (so not immutable) but its useable in EWS.

Cheers
Glen

I am caching some information fetched via EWS and need to invalidate the cache if a different user gets the same email address.

Then I would use MailboxDN and you can then use Autodiscover to validate the PrimarySMTP against the MailboxDN.

Cheers
Glen