From Forum Nokia Wiki

CMsvEntry* entry = CMsvEntry::NewL(*iMsvSession, KMsvGlobalInBoxIndexEntryId, TMsvSelectionOrdering());
CleanupStack::PushL(entry);	
CMsvEntrySelection* entries = entry->ChildrenL(); 
CleanupStack::PushL(entries);
TInt nUnreadCount;
for(TInt i=0;i<entries->Count();i++) 
   {		
    entry->SetEntryL(entries->At(i));
    TMsvEntry msvEntry(entry->Entry());
    if( msvEntry.Unread())
       nUnreadCount++;
   }
CleanupStack::PopAndDestroy(2);