Configuring and running postfix (Mojave 10.14.4) I've gone through all the questions (and answers) about postfix on AskDifferent and come up empty so far.

(~) postconf -n
biff = no
command_directory = /usr/sbin
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
debugger_command = PATH=/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin ddd $daemon_directory/$process_name $process_id &amp; sleep 5
header_size_limit = 4096000
html_directory = /usr/share/doc/postfix/html
inet_interfaces = loopback-only
inet_protocols = all
mail_owner = _postfix
mailbox_size_limit = 0
mailq_path = /usr/bin/mailq
manpage_directory = /usr/share/man
message_size_limit = 10485760
mynetworks = 127.0.0.0/8, [::1]/128
newaliases_path = /usr/bin/newaliases
queue_directory = /private/var/spool/postfix
readme_directory = /usr/share/doc/postfix
recipient_delimiter = +
relayhost = [smtp.sendgrid.net]:587
sample_directory = /usr/share/doc/postfix/examples
sendmail_path = /usr/sbin/sendmail
setgid_group = _postdrop
smtp_sasl_auth_enable = yes
smtp_sasl_mechanism_filter = AUTH LOGIN
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noplaintext,noanonymous
smtp_sasl_tls_security_options = noanonymous
smtp_tls_security_level = encrypt
smtp_use_tls = yes
smtpd_client_restrictions = permit_mynetworks permit_sasl_authenticated permit
smtpd_tls_ciphers = medium
tls_random_source = dev:/dev/urandom
unknown_local_recipient_reject_code = 550


The postconf output is in line with everything in response to similar questions on AskDifferent.

(~) sudo postfix start
postfix: Postfix is running with backwards-compatible default settings
postfix: See http://www.postfix.org/COMPATIBILITY_README.html for details
postfix: To disable backwards compatibility use "postconf compatibility_level=2" and "postfix reload"
postfix/postfix-script: starting the Postfix mail system
postfix/postfix-script: fatal: mail system startup failed


I don't know where to go from here, and am seeking references to docs or specific suggestions.
Try this command:

postconf compatibility_level=2


Followed by a reload:

postfix reload


I don't know why this fixed the problem, but it did. I'd love to understand why this was not default and why it matters.