The "pay" sheet music provided through the first link mentioned in this thread says that it's for choral groups, so one would hope it provides good four-part harmony. Whether it's "the" authentic harmony from the Phil Spector arrangement -- well, that's another question.

If I may creep just a bit, let me venture the opinion that "Please Mr. Postman" is an especially interesting rock-vocal-harmony piece. The background-singers' part has different lyrics and a different rhythmic pattern for every verse, building in complexity from beginning to end. I suppose that this song is not, strictly speaking, a "doo-wop" number; "Motown girl-group" is probably a more accurate classification.