Capitalizing the pronoun when it is referring to the person/people to whom a communication is directed is normally used in commercial/business communications. Capitalizing it would mean to consider the receiver of the communication important.
Nowadays, it is less and less used; somebody could also find it affected. Eventually, the communication can start with Gentile Cliente, but then the pronoun or the possessive is written normally as any other word (which means it is capitalized only at the beginning of a sentence.)
In any case, it is not a matter of grammar; It just a way to convey a meaning ("you, the listener, are important for us") that otherwise would require using adjectives (e.g. stimato).