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.
I believe you still have to stop at certain points to check the responses. You might not want to send a message if one of the recipients is invalid, but only that command will return an error code, and the message sending will succeed.
You'd also need to prevent command injection. If the response code to DATA is an error, but you sent the message anyway, the whole message body will be interpreted as commands. Oops! The line ending bug (SMTP smuggling discovered early this year) was bad enough.
