sm said:
If the DNS server is slow, it will cause problems. If you are going to use DNS based blacklists, you should have a reliable DNS server.
While I generally agree with this statement, I would add that that reliability and speed are not always a "yes/no", or "good/bad" thing. There are some grey areas or varying degrees of responsiveness and speed and, definitely, using blacklists in the way that SURBL does puts a new level of burden or stress on DNS servers.
For example, if a DNS server is generally good, but does have a 12 millisecond response time (using an arbitrary number) due to being shared among dozens (or hundreds) of other servers, this 12 milliseconds gets multiplied out when a dozen SURBL lookups hit at practically the same instant. In this case, the message is dependent on ALL of these requests being answered before the message can continue. This can also cause the mail server to have to work more threads at any one given time... which can lead to additional scalability and performance issues.
Therefore, many DNS server situations which are more than adequate for regular situations may not be adequte for SURBL lookups.
Rob McEwen