Confused about DNS issue

[The Other Mike]

Trying to diagnose an email issue to a certain domain.   Doing an
NSLOOKUP for the MX record on this particular domain it comes back
with a server name (say server.domain.com).  Now, when I ping
server.domain.com, it does not resolve an IP address.  I did this from
our west coast office and from my home PC on the east coast (different
ISP's) and neither resolve an IP.  But...if I go to www.dnsstuff.com
and use their ping, it does resolve an IP.  Also if I do a ping -a
using the IP from dnsstuff on my home PC or work PC, it will
successfully do the reverse lookup and return server.domain.com.  Can
someone enlighten me as to what might be going on??   Why can I get
the reverse lookup but no host resolution using 2 different
ISP's...and why does dnsstuff.com get both with no problem??