Postfix no longer automatically appends the system default CA certificates. When pipe-to-command delivery fails with a signal, mail is now correctly deferred, instead of being returned to sender. Poor `smtpd_proxy_filter` TCP performance over loopback (127.0.0.1) connections was fixed by adapting the output buffer size to the MTU. The SMTP server no longer applies the `reject_rhsbl_helo` feature to non-domain forms such as network addresses. The Postfix SMTP server failed to deliver a "421" response and hang up the connection after Milter error.
