At one point there were formal and informal second person pronouns (informal: thou, thee, etc; formal: you, ye, etc.) However, "[b]y the late seventeenth century you had become normal in almost all contexts and thou and thee were limited to the Bible and religious use, the Quakers, and regional dialects" according to the OED Blog. (More on this can be found here.)
Thus, the most formal second person (singular and plural) pronoun is you (in contrast, there are currently several widely-used informal second person pronouns in different regions such as "y'all" and "you guys"). You can see you used in examples of letters/emails in pretty much any source you pick up. Here's an example from a book (Technical Communication, page 369) I happen to own (emphasis added):
Dear Mr. Larsyn:
As steady customers of yours for over 15 years, we came to you first when we needed a quiet pile driver for a job near a residential area. On your recommendation, we bought your Vista 500 Quiet Driver[...]
Jack Robbins, President
Use of "you" can create an informal tone in some types of writing, such as essay writing, documentation, or academic writing. In other words, writing which is not addressed to anyone where it would make sense to use "one" instead of "you" or rewrite things entirely.
And of course, there are other (unrelated) strategies to make writing more (or less formal), such as not using contractions.