Very true. Is it the mydomain_fallback entry?
postconf -n
biff = no
command_directory = /usr/sbin
config_directory = /etc/postfix
content_filter = smtp-amavis:[127.0.0.1]:10024
daemon_directory = /usr/libexec/postfix
debug_peer_level = 2
enable_server_options = yes
header_checks = pcre:/etc/postfix/custom_header_checks
html_directory = /usr/share/doc/postfix/html
inet_interfaces = all
local_recipient_maps =
mail_owner = _postfix
mailbox_size_limit = 0
mailbox_transport = dovecot
mailq_path = /usr/bin/mailq
manpage_directory = /usr/share/man
maps_rbl_domains =
message_size_limit = 20971520
mydestination = $myhostname, localhost.$mydomain, mail.dmimage.ca, server.dmimage.ca, $mydomain
mydomain = dmimage.ca
mydomain_fallback = localhost
myhostname = mail.dmimage.ca
mynetworks = 127.0.0.0/31,192.168.6.0/24
newaliases_path = /usr/bin/newaliases
queue_directory = /private/var/spool/postfix
readme_directory = /usr/share/doc/postfix
recipient_delimiter = +
relayhost =
sample_directory = /usr/share/doc/postfix/examples
sendmail_path = /usr/sbin/sendmail
setgid_group = _postdrop
smtpd_client_restrictions = permit_mynetworks permit_sasl_authenticated hash:/etc/postfix/smtpdreject cidr:/etc/postfix/smtpdreject.cidr reject_rbl_client zen.spamhaus.org permit
smtpd_enforce_tls = no
smtpd_helo_required = yes
smtpd_helo_restrictions = reject_invalid_helo_hostname reject_non_fqdn_helo_hostname
smtpd_pw_server_security_options = cram-md5
smtpd_recipient_restrictions = permit_sasl_authenticated permit_mynetworks reject_unauth_destination check_policy_service unix:private/policy permit
smtpd_sasl_auth_enable = yes
smtpd_tls_CAfile = /etc/certificates/server.dmimage.ca.chain.pem
smtpd_tls_cert_file = /etc/certificates/server.dmimage.ca.cert.pem
smtpd_tls_exclude_ciphers = SSLv2, aNULL, ADH, eNULL
smtpd_tls_key_file = /etc/certificates/server.dmimage.ca.key.pem
smtpd_tls_loglevel = 0
smtpd_use_pw_server = yes
smtpd_use_tls = yes
tls_random_source = dev:/dev/urandom
unknown_local_recipient_reject_code = 550
virtual_alias_maps =