I was so impressed, I still have the envelope. Just for grins here is exactly how it was addressed:
Mr. Frederick Mattox
(from Costa Mesa, California, U.S.A.)
Guest on Veendam
Cruise Ship of "Holland America"
Docked at Arhus Port
on July 1st, 2007
ARHUS - DENMARK
And the cabin number was on the envelope in another hand, presumably someone in the Purser's staff.
So it had all the necessary information. It was postmarked June 14, 2007 from Italy.
Still, rather remarkable.