Use the following command at command window to get list of registered Event sink registration:

Cscript [//NOLOGO] ListMailboxes.vbs {/F:filename | /S:servername} [/DEBUG]

Where,

//NOLOGO

Suppresses the version of Windows Script Host (WSH) and copyright information.

/F

The name of the text file that contains the names of the servers, will be available one per line, for which you want to list mailboxes. You must create the text file before you use this script.

/S

Name of the server that contains the mailboxes that you want to list.

/DEBUG

Outputs verbose debug information that you can use for diagnosis.

Note:

You must specify either the /F or /S switch.

For more information, please read this Technet article click here.