The "you" replaces the account's or the identity's email, but not across multiple accounts.
Not sure what you mean with "not across multiple accounts" ?
I use three accounts, all on the same domain, but using different names before the @ (dave@, postmaster@, webtrees@...). All show as "you" (only the colour triangles in the smart folder are different).
Can I add a bit more detail... It seems that the word "you" only replaces an exact match. If, like me, you also use a "catch all" mailbox, then on that account, whatever the sender wrote is in the header, unless it also matches the account name. My catch all account is "postmaster@mydomain.com". If an email is sent to "postmaster@..." then it is shown as "You". If an email is sent to "randomname@...", then it still goes in the postmaster account, but shows "randomname@..."
Now I can think of another valid reason to ask that "you" might be suppressed, even though I am happy with it most of the time. If you have a printer app, and wish to print out an email for some business purpose, it is unhelpful if it does not show who "you" is.
