Data in the Directory Data Service (DDS) source has changed and needs to be immediately updated in Messaging Gateway (SMG) rather than waiting for the default cache timeout.
Example
Recipient validation is rejecting mail to a specific recipient because the recipient's email address was mistyped in Active Directory. Messaging Gateway has already cached the recipient address as invalid due to previous failed lookups. Clearing the DDS cache will force SMG to requery the DDS source rather than continue to operate from the cached information.