I've upgraded my Postfix mail system from opendkim to dkimpy in Debian Stretch using Backports, because I'm trying to implement ARC signing and verification. I was able to get dkimpy working in so far that DKIM verification was done on inbound mail, but couldn't get DKIM signing to work and found little to no information / documentation on ARC in dkimpy. Any help would be appreciated. My config below.
# /etc/postfix/master.cf
smtp inet n - y - - smtpd
-o smtpd_milters=inet:localhost:8892
-o milter_macro_daemon_name=VERIFYING
submission inet n - y - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_milters=inet:localhost:8892
-o non_smtpd_milters=inet:localhost:8892
-o milter_macro_daemon_name=ORIGINATING
# /etc/dkimpy-milter.conf
Domain *
KeyFile /etc/pki/dkim/alpha.private
Selector alpha
Canonicalization relaxed/simple
Mode sv
Socket inet:8892@localhost
PidFile /var/run/dkimpy-milter/dkimpy-milter.pid
UserID dkimpy-milter
MacroList daemon_name|ORIGINATING
MacroListVerify daemon_name|VERIFYING