How do you use NSLOOKUP with MX records?

At the command prompt type "NSLOOKUP <enter>".
This will give you a ">" prompt.
Type "set type=MX <enter>".
Type "simpledns.plus <enter>".
You should now get the MX records for "simpledns.plus".

An easier alternative (which also works on Win9x) would be to use the DNS Look Up tool in Simple DNS Plus.