Unify all smtplib commands to be uppercase

Hello,

I know there was a previous issue made years ago about smtplib being inconsistent with uppercase and lowercase commands, however the succeeding fix made the commands lowercase. I think these should be switched to uppercase and here's why:

RFC 5321 states "A few SMTP servers, in violation of this specification (and RFC 821) require that command verbs be encoded by clients in upper case. Implementations MAY wish to employ this encoding to accommodate those servers."

Servers such as Postfix and many others have resolved to simply making all SMTP commands and options uppercase as depicted (though not required) in all the RFC documents. Though it is not strictly necessary, it will have better support (even if marginally) and is a quick fix.

Also, smtplib seems to already send some of these commands and options like STARTTLS and SMTPUTF8 uppercase which technically leaves the inconsistency of the last issue unsolved.

Ultimately, I think it would be best for posterity to simply make all of these uppercase and it should be a quick fix.

I can make the proper edits and make a pull request if there isn't some obvious reason for this I'm missing.

Let me know your thoughts! 🙂

Linked PRs

• gh-136733

Revisa las rutas de comandos de smtplib y las pruebas relacionadas para identificar cada comando y opción cuya combinación de mayúsculas y minúsculas sea incoherente. El cambio estará completo cuando todos los comandos de smtplib utilicen verbos en mayúsculas y las pruebas existentes pasen; ten en cuenta que el PR enlazado gh-136733 indica que el trabajo ya está en curso.