The Internet Mail Connector (IMC) caches all successfull DNS lookups, to a maximum of 1,000 entries. This prevents DNS load-balancing from working.

You can turn off IMC's caching by navigating to:

HKLM\System\CurrentControlSet\Services\MSExchangeIMC

Add Value name DestinationDomainThreshold as a type REG_DWORD and set the value to 0. The default is 1,000.