Postfix + SASL = "Relay Access Denied" en cas d'envoi depuis l'extérieur du réseau

J'ai dans /etc/postfix/main.cf :

smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination

et SASL semble fonctionner :

$ testsaslauthd -u yang -p ... -f /var/spool/postfix/var/run/saslauthd/mux -s smtpd
0: OK "Success."

mais j'obtiens "Relay access denied" lorsque je me connecte depuis l'extérieur du réseau (alors que localhost, par exemple, fonctionne toujours très bien) :

$ telnet blah.com 25
auth plain ...
235 2.7.0 Authentication successful
mail from:<yang@blah.com>
250 2.1.0 Ok
rcpt to:<yang@dest.com>
554 5.7.1 <yang@dest.com>: Relay access denied

Journaux :

Oct 18 21:10:19 blah postfix/smtpd[13882]: connect from unknown[x.x.x.x]
Oct 18 21:10:19 blah postfix/smtpd[13882]: setting up TLS connection from unknown[x.x.x.x]
Oct 18 21:10:19 blah postfix/smtpd[13882]: Anonymous TLS connection established from unknown[x.x.x.x]: TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)
Oct 18 21:10:19 blah postfix/smtpd[13882]: NOQUEUE: reject: RCPT from unknown[x.x.x.x]: 554 5.7.1 <yang@dest.com>: Relay access denied; from=<yang@blah.com> to=<yang@dest.com> proto=ESMTP helo=<[y.y.y.y]>
Oct 18 21:10:19 blah postfix/smtpd[13882]: disconnect from unknown [x.x.x.x]

Cela fait un certain temps que je ne sais plus où j'en suis. Un indice ?

Ma configuration complète suit ; elle est basée sur https://help.ubuntu.com/community/Postfix .

/etc/default/saslauthd :

START=yes
PWDIR="/var/spool/postfix/var/run/saslauthd"
PARAMS="-m ${PWDIR}"
PIDFILE="${PWDIR}/saslauthd.pid"
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

/etc/postfix/main.cf :

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

# TLS parameters
smtp_tls_loglevel=1
smtp_tls_security_level=may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_tls_cert_file=/etc/ssl/certs/blah.crt
smtpd_tls_key_file=/etc/ssl/private/blah.key
smtpd_tls_loglevel=1
smtpd_tls_security_level=may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache

smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = blah.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = localhost.blah.com, localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all

# From <http://www.postfix.org/VIRTUAL_README.html>

virtual_mailbox_domains = blah.com invalid.invalid
virtual_mailbox_base = /var/mail/blah
virtual_mailbox_maps = hash:/etc/postfix/vmailbox
virtual_alias_maps = hash:/etc/postfix/valiases
virtual_minimum_uid = 100
virtual_uid_maps = static:1001
virtual_gid_maps = static:1001

sender_bcc_maps = hash:/etc/postfix/bccmaps

# DKIM
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

# Enforce SPF
smtpd_recipient_restrictions =
  permit_mynetworks
  reject_unauth_destination
  check_policy_service unix:private/policyd-spf
policyd-spf_time_limit = 3600

/etc/postfix/sasl/smtpd.conf :

pwcheck_method: saslauthd
mech_list: plain login