Apologies if you already know this, but pipelining has existing in SMTP for decades now. You can see if an SMTP server supports it when using EHLO (instead of HELO).

  Trying 74.125.202.26...
  Connected to smtp.google.com.
  Escape character is '^]'.
  220 mx.google.com ESMTP 8926c6da1cb9f-489374a766fsi10850940173.171 - gsmtp
  EHLO testing
  250-mx.google.com at your service, [47.227.77.52]
  250-SIZE 157286400
  250-8BITMIME
  250-STARTTLS
  250-ENHANCEDSTATUSCODES
  250-PIPELINING   <============= HERE
  250-CHUNKING
  250 SMTPUTF8

I believe this RFC 2920 is the original standard going back to 2000. I remember pipelining being a thing in Postfix about 20 years ago, at least.