Wednesday, January 29, 2014

The famous 9646 error on Exchange servers

Most Exchange administrators have at least one time seen an error with Event ID 9646 and some seen this many times.

An example might look like this.
Mapi session /o=Exchange_orgname/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=user with client type MOMT exceeded the maximum of 500 objects of type Folder.

another example might end with this:
.. exceeded the maximum of 250 object of type Folder View.

Why does this happen?
Exchange has some built-in thresholds for various things that try to stop bad user behavior or a client behaving badly to not use all resources on the server to make other users suffer.
You can try to change client configuration for users experiencing this problems by changing cached mode on/off and also for extra mailboxes opened, outlook add-ons might also cause this. upgrading clients to newer version might also help, but sometime you cant change what users do and you must change limits instead.

How to change limits
You change limits by registry values and Microsoft has a TechNet article that describe these values Exchange Store Limits. The key here is to read the error text and find what threshold exceeded its limit, then read the TechNet article and implement a new threshold by editing registry values.
One problem here is to decide what limits you should set, unfortunately you have to set a value and then see if its working or not.
To have it all working you must restart Exchange services so don’t count on this working until a restart is done.

Article don’t cover Exchange server 2013 but the same limits apply and also the same solution to add/edit registry values on your mailbox servers.