Postfix limit incoming or receiving email rate

Postfix (smtpd daemon) can enforce a number of limits on incoming email. This will stop email flooding attacks.

A bot connects to your Postfix email server and sends garbage commands or spam, attempting to crash your server. You can limit:

=> The length of lines in a message and so on

=> The size of messages

=> The number of recipients for a single delivery

Try following directives in your postfix main.cf config file:
smtpd_error_sleep_time – The SMTP server response delay after a client has made more than $smtpd_soft_error_limit errors, and fewer than smtpd_hard_error_limit errors, without delivering mail.
smtpd_soft_error_limit : The number of errors a remote SMTP client is allowed to make without delivering mail before the Postfix SMTP server slows down all its responses.
smtpd_hard_error_limit : The maximal number of errors a remote SMTP client is allowed to make without delivering mail. The Postfix SMTP server disconnects when the limit is exceeded.

Open config file
# vi main.cf
Append following directives:
smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 10
smtpd_hard_error_limit = 20

Save and restart/reload postfix configuration
# /etc/init.d/postfix restart

Postfix waits one second before each error such as HELO command not provided or FQDN hostname does not exists etc After 10 such errors postfix will start to increase delay. If error limits touches 20 Postfix will disconnect client.

Source http://www.cyberciti.biz

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