Tarpitting seems to stump most of us when searching through the knowledge base for how to reduce the prevalence of Directory Harvesting attacks.  Tarpitting is the practice of deliberately inserting a delay into certain SMTP communications that are associated with spam or with other unwanted traffic. To be effective, these kinds of communications typically rely on generating a high volume of traffic. By slowing an SMTP conversation, you can dramatically reduce the rate at which automated spam can be sent or at which a dictionary attack can be conducted.

To enable tarpitting
The tar pit feature can be enabled and configured by setting a registry key. To do this, follow these steps.
Note: If the TarpitTime registry entry does not exist, Exchange behaves as if the value of this registry entry were set to 0. When the registry entry has a value of 0, there is no delay when the SMTP address verification responses are sent.

1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and then click to select the following registry subkey:
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type TarpitTime as the registry entry name, and then press ENTER. 
5. On the Edit menu, click Modify.
6. Click Decimal.
7. In the Value data box, type the number of seconds that you want to delay SMTP address verification responses for each address that does not exist. Then, click OK. For example, type 5, and then click OK. This delays SMTP address verification responses for 5 seconds.
8. Quit Registry Editor.
9. Restart the Simple Mail Transport Protocol (SMTP) service.

There's a good explanation about Tarpitting over on the Exchange team blog, and you can read more about Recipient filtering in the knowledge base article and also at David's blog...