The original MCVAX was the machine (a Digital VAX 11/780, serial number 38) which brought Internetworking to Europe. It was located in the Mathematical Centre in Amsterdam (now CWI) and was run by the legendary Piet Beertema.
Mcvax was originally replaced by mcsun (a Sun 4/280) and by more newer, even more powerful systems since then. The mcvax.org domain lives on in memory of all those machines. The current mcvax.org machine has nothing whatsoever to do with European networking, doesn't act as a major mail relay, and is administered by James Aldridge in his limited spare time.