Klein. "The 'v' is written in lower case, except when the surname is used as standalone (when the first name or initials are omitted), in which case it is capitalized, as in 'de schilder Van Gogh ('the painter Van Gogh)." (
http://en.wikipedia.org/wiki/Van_%28Dutch%29)
Ami is writing to a Dutchman, who will be accustomed to seeing
van in lower case and will view
Van as an incorrect spelling of his name.
In addition to that, even for correct US usage, the
Chicago Manual of Style says,
"In English usage, the particles
van,
van den,
ter, and the like are lowercased when full names are given but usually captialized when only the last name is used.
"Joannes van Keulen; Van Keulen
Pieter van den Keere; Van den Keere
Vincent van Gogh; Van Gogh."
I am not sure, but I would imagine that the salutation should be:
Dear Mr. van Mustermann. To avoid the problem:
Dear Sir.