mcvax.cwi.nl used to be the international backbone node of EUnet, the European Unix network. It was located in Amsterdam, Netherlands and belonged to "Centrum voor Wiskunde en Informatica" (Centre for Mathematics and Computer Science) which is an institute belonging to a foundation called "Mathematisch Centrum". Since the first mcvax was on of the first VAXen in Europe and one of it's first uucp connections was to a machine called decvax it was quickly christened mcvax. Some also say this was done to give Jim McKie a nice mail address: mcvax!mckie. But this is certainly not true at all. The function of EUnet international backbone moved to another VAX later but the name moved with it, because in those days of mainly uucp based mail and before widespread use of pathalias it was simply not feasible to rename the machine to "europa" as was suggested at one stage.
Mcsun (or relay.eu.net or net.eu.relay in some parts of Europe) replaced the international backbone host of EUnet around 1990. This machine was donated by Sun Microsystems owned by the European Unix Systems User Group (EUUG). It was located about 5m from where mcvax used to be and operated by the same people.
Mcvax has finally ceased to exist in the domain and uucp namespaces. It still exists in the EARN/BITNET namespace.
[Posting by Daniel Karrenberg