Is there a formal version of "E tu"?
Is there a formal version of "E tu"? Or would you say "E voi"?
The custom of addressing politely people using the pronoun Lei started during the age of Renaissance. But since the dawn of the Italian language, people of respect were addressed with the plural pronoun Voi.
The introduction of the Lei scheme did not replace the older one; so for several centuries both ways of addressing listeners coexisted, with a prevalence of the older one (Voi) in the southern areas, and of the new one (Lei) elsewhere.
From 1938 to 1943 the use of Lei was officially banned, so the use of Voi became forcibly widespread. Then, within a decade or so, the previous customs were gradually restored.
Today the use of Voi is completely alien to northen Italian speakers. It is almost extinct in the central regions; a few elderly speakers, mostly in rural areas, still use it, but it is dying out very rapidly. Instead, it is still very common in the south, particularly in Naples and its surroundings.
Interestingly, here in Rome the Voi scheme was the traditional one up to the late 1800s. The Lei scheme became popular among the middle and upper classes since the early 1900s, but many dialect-speaking commoners kept preferring the Voi scheme up to the 1950s, when the use of Lei gradually took over. Now it is very rarely heard, even among elderly dialect speakers.
In most sound films up to the mid 1950s the Voi scheme is constantly used, as well as in modern films set in the past.
Did Mussolini (1938-1943) not like the potential confusion with the feminine form?
No, the main reason is that the Lei form was considered "a trait of bourgeois snobbishness". So people were invited (...or better forced) to use the plain tu form also with strangers, and to reserve the Voi form only for those held in high esteem. The people simply switched to the use of Voi instead of Lei, because using the tu form with strangers sounds impolite and would have been embarassing. Several intellectuals took position against these new guidelines. Some though approved them. As soon as the regime fell, most people started using again the Lei form. But it took about a decade before the radio, the movies, etc. got completely rid of the use of Voi.
Formal version is "E Lei" in standard Italian. In southern dialects you might also hear "E voi", but that's just dialects. "E voi" is used normally as plural you, both formal and informal, just as Dcarl1 noted.