My post mail server is used for spam

HI,


I'm a little in double because my postfix server is used to send an
huge amount of spam, generating huge logs like that :

postfix/error[2120]: 993AE145D: to=<xbeefon@yahoo.com.tw>, relay=none,
delay1, delays0/0.07/0/0.31, dsn=4.7.0, status=deferred
(delivery temporarily suspended: host
mx1.mail.tw.yahoo.com[203.188.197.119] refused to talk to me: 421
4.7.0 [TS01] Messages from 62.161.100.158 temporarily deferred due to
user complaints - 4.16.55.1; see
http://postmaster.yahoo.com/421-ts01.html)

I'm running squeeze, my account are secured with strong password, town
can I stop that ?

thanks,

Here's my postfix main.cnf :


#myorigin = /etc/mailname
smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no
# appending .domain is the MUA's job.
append_dot_mydomain = no
# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h
readme_directory = no
# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous
smtp_tls_note_starttls_offer = yes
tls_random_source = dev:/dev/urandom
smtp_tls_scert_verifydepth = 5
smtpd_tls_ask_ccert = yes
smtpd_tls_req_ccert =no
smtp_tls_enforce_peername = no
# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.
myhostname = VOLTALIAMSG.voltalia.local
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = voltalia.com VOLTALIAMSG.voltalia.local,
localhost.voltalia.local, localhost
#relayhost =  [smtp.fr.oleane.com]:587
#relayhost = [smtp.gmail.com]:587
mynetworks = 192.168.150.0/24 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtpd_sasl_local_domain = $myhostname
home_mailbox = Maildir/
virtual_alias_maps = hash:/etc/postfix/virtual


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