Linux postfix/dovecot 554 Relay access denied

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http://stackoverflow.com – I have this error 554 Relay access denied when trying to send email from my outlook client. I can read incoming mails but cannot send. If i connect with telnet localhost 25 i can send external emails, but with outlook client it doesn't work. Here's my postfix and dovecot config : postconf -n alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases append_dot_mydomain = no biff = no config_directory = /etc/postfix inet_interfaces = all mailbox_size_limit = 0 mydestination = localhost myhostname = mail.mydomain.com mynetworks = [::ffff:]/104 [::1]/128 myorigin (HowTos)