SMTP pipelining was actually the cause of fairly recently discovered vulnerability. https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwid

SMTP pipelining was actually the cause of fairly recently discovered vulnerability. https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/ https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-... https://www.postfix.org/smtp-smuggling.html https://www.postfix.org/smtp-smuggling.html Probably the main reason it is recommended against.