ajuda log postfix

boa tarde

Alguém poderia me dar uma ajuda na interpretação deste log do postfix:

Aug 22 13:40:49 mail postfix/smtpd[1790]: warning: 196-215-38-192.dynamic.isadsl.co.za[196.215.38.192]: SASL LOGIN authentication failed: authentication failure
Aug 22 13:42:00 mail postfix/smtpd[1790]: message repeated 19 times: [ warning: 196-215-38-192.dynamic.isadsl.co.za[196.215.38.192]: SASL LOGIN authentication failed: authentication failure]
Aug 22 13:42:02 mail postfix/smtpd[1790]: too many errors after AUTH from 196-215-38-192.dynamic.isadsl.co.za[196.215.38.192]

Pelo que entendi houve 19 tentativas de autenticação e o Sasl falhou... teria alguma maneira de bloquear isso? isso representa alguma ameaça?

abraço

Amigo, posta o main.cf desta forma todos do fórum poderão lhe ajudar melhor.

ok, segue meu main.cf:


# 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



smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
strict_rfc821_envelopes = yes
# a linha acima exige o envelopamento por <> dos emails
unknown_local_recipient_reject_code = 500
unknown_address_reject_code = 554
unknown_hostname_reject_code = 554
unknown_client_reject_code = 554
# as 4 linhas acima bloqueiam remetentes desconhecidos
myhostname = mail.xxxxxxxx.com.br
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = xxxxxx.com.br, mail, localhost.localdomain, localhost
relayhost =
mynetworks = 127.0.0.0/8, localhost
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
mydomain = xxxxx.com.br
home_mailbox = Maildir/

milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:127.0.0.1:8891,inet:127.0.0.1:8892
non_smtpd_milters = inet:127.0.0.1:8891,inet:127.0.0.1:8892

# SASL smtp-auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_local_domain =
smtpd_sender_restrictions = permit_sasl_authenticated, permit_mynetworks, permit

# POP Before SMTP
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination