I think it is improper professional language. You all is another way of saying "y'all", which someone would never write in a business letter.

The problem is that there is an inconsistency in the english language in that there is no plural for "you". "You" is both singular and plural. He or she becomes they or them.

That is why in the South, y'all is used (or even "all y'all"), and in other parts of the country, "you guys" is used.

There is no need for either because you is already plural.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".