Huh that's really interesting. I wonder if it would make sense for a mail server to always respond OK to RCPT, but respond with failure to DATA if any recipient

Huh that's really interesting. I wonder if it would make sense for a mail server to always respond OK to RCPT, but respond with failure to DATA if any recipients are undeliverable? There would be a slight change in semantics here (there would be no way for the sending SMTP server to know which subset of the recipients are undeliverable) though, which might make it hard to notify users about specific bouncing addresses.