weird problem with lame delegations

[Will Yardley]

I've been looking through our DNS querylogs recently (in the process of
tracking down a couple of weird problems), and I've noticed something
sort of curious.... certain DNS clients seem to send a TON of requests
to our nameservers if a lame delegation exists (see below). Is this
because a SERVFAIL message is being sent and the client keeps retrying
over and over?

If I setup a bogus zone with a long TTL, this problem goes away.

Also, in general, what's considered best practice for a registrar who is
also a host when a customer removes hosting services for a domain but
still has nameservers pointing to you... 

I assume the best thing would be to point the domain to some sort of
generic informational page, and maybe have an MX pointing to a server
that just rejects the mail?

For example, just a second's worth:

Sep 03 14:13:27.166 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.167 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.167 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.168 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.168 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.169 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.415 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.416 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.417 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.417 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.418 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.418 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:27.962 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.963 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.963 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.963 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.964 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.964 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:27.965 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.417 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.418 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.418 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.419 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.419 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.419 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.420 client 62.138.229.218#37098: query: ferdartor.net IN A
Sep 03 14:13:28.963 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.967 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.968 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.968 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.969 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.969 client 62.138.229.218#37626: query: ferdartor.net IN A
Sep 03 14:13:28.970 client 62.138.229.218#37626: query: ferdartor.net IN A