Thanks for the reply. That's all I wanted to know. Some of our mail is not being received by other mail servers because they claim the cannot do a reverse lookup of our domain, yet I have tested this successfully.
DNS Query uses by default the UDP protocol. This is used for normal queries. However, if a response can't fit into one single UDP packet - take note that the maximum payload is 512 bytes as defined by RFC1035 - the resolver must switch to the TCP protocol. Because this will always be the case for zone transfers, that terminology is used in ISA server.
Keep in mind that this is *not* the only case the TCP protocol will be used. This happens also very often with an MX record lookup. Moreover, I believe that the SMTP implementations by Microsoft (IIS and Exchange) always try to do the MX record lookups with the TCP protocol first.