Post fix Hardening – configure postfix with anti spam blacklisting.

Postfix is free and powerful MTA. You can easily configure Postfix to block spam. You need to add
following directives to /etc/postfix/main.cf file:

=> disable_vrfy_command = yes : Disable the SMTP VRFY command. This stops some techniques used to harvest email addresses.

=> smtpd_delay_reject = yes
It allows Postfix to log recipient address information when rejecting a client name/address or sender address, so that it is possible to find out whose mail is being rejected.

=> smtpd_helo_required = yes
Require that a remote SMTP client introduces itself at the beginning of an SMTP session with the HELO or EHLO command. Many spam bot ignores HELO/EHLO command and you save yourself from spam. Following lines further restrictions on HELO command:
smtpd_helo_restrictions = permit_mynetworks,
reject_non_fqdn_hostname, Reject email if remote hostname is not in fully-qualified domain form. Usually bots sending email don’t have FQDN names. reject_invalid_hostname, Reject all bots sending email from computers connected via DSL/ADSL computers. They don’t have valid internet hostname.
permit

You can put the following access restrictions that the Postfix SMTP server applies in the context of the RCPT TO command.
=> smtpd_recipient_restrictions =
reject_invalid_hostname, – Reject email if it not valid hostname
reject_non_fqdn_hostname, – Reject email if it not valid FQDN
reject_non_fqdn_sender, – Reject the request when the MAIL FROM address is not in fully-qualified domain form. For example email send from xyz or abc is rejected.
reject_non_fqdn_recipient, – Reject the request when the RCPT TO address is not in fully-qualified domain form
reject_unknown_sender_domain, – Reject email, if sender domain does not exists
reject_unknown_recipient_domain, Reject email, if recipient domain does not exists
permit_mynetworks,
reject_rbl_client list.dsbl.org, Configure spam black lists
reject_rbl_client sbl.spamhaus.org,
reject_rbl_client cbl.abuseat.org,
reject_rbl_client dul.dnsbl.sorbs.net,
permit

Open /etc/postfix/main.cf file :
# vi /etc/postfix/main.cf

Set/modify configuration as follows

disable_vrfy_command = yes
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_helo_restrictions = permit_mynetworks,
reject_non_fqdn_hostname,
reject_invalid_hostname,
permit
smtpd_recipient_restrictions =
permit_sasl_authenticated,
reject_invalid_hostname,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_sender_domain,
reject_unknown_recipient_domain,
permit_mynetworks,
reject_rbl_client list.dsbl.org,
reject_rbl_client sbl.spamhaus.org,
reject_rbl_client cbl.abuseat.org,
reject_rbl_client dul.dnsbl.sorbs.net,
permit
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20

Save and close the file. Restart postfix:
# /etc/init.d/postfix restart

Watch out maillog file. Now you should see lots of spam email blocked by above configuration directive:
# tail -f /var/log/maillog
Output:

Jan 9 06:07:22 server postfix/smtpd[10308]: NOQUEUE: reject: RCPT from 183-12-81.ip.adsl.hu[81.183.12.81]: 554 Service unavailable; Client host [81.183.12.81] blocked using dul.dnsbl.sorbs.net; Dynamic IP Addresses See: http://www.sorbs.net/lookup.shtml?81.183.12.81; from= to= proto=ESMTP helo=<183-12-230.ip.adsl.hu>
Jan 9 06:07:23 server postfix/smtpd[10308]: lost connection after RCPT from 183-12-81.ip.adsl.hu[81.183.12.81]
Jan 9 06:07:23 server postfix/smtpd[10308]: disconnect from 183-12-81.ip.adsl.hu[81.183.12.81]
Jan 9 06:10:43 server postfix/anvil[10310]: statistics: max connection rate 1/60s for (smtp:81.183.12.81) at Jan 9 06:07:17
Jan 9 06:10:43 server postfix/anvil[10310]: statistics: max connection count 1 for (smtp:81.183.12.81) at Jan 9 06:07:17
Jan 9 06:10:43 server postfix/anvil[10310]: statistics: max cache size 1 at Jan 9 06:07:17
Jan 9 06:16:58 server postfix/smtpd[10358]: warning: 81.92.197.249: address not listed for hostname unassigned.or.unconfigured.reverse.nfsi-telecom.net
Jan 9 06:16:58 server postfix/smtpd[10358]: connect from unknown[81.92.197.249]
Jan 9 06:17:00 server postfix/smtpd[10358]: NOQUEUE: reject: RCPT from unknown[81.92.197.249]: 550 : Recipient address rejected: User unknown in virtual alias table; from=<> to= proto=ESMTP helo=
Jan 9 06:17:00 server postfix/smtpd[10358]: disconnect from unknown[81.92.197.249]

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