Postfix smtpd_client_restrictions with check_client_access not working

I suddenly receive many spam mails per day and I’m trying to filter the source by IP.

In my main.cf file I have this:

smtpd_client_restrictions = 
        check_client_access cidr:/etc/postfix/blacklist,
        permit

and in this blacklist file I have IP addresses and networks defined in the following way according to https://linux.die.net/man/5/access. I don’t filter by domain name since every spam has a different domain name.

1.2.3.4  REJECT Blacklisted
5.6.7    REJECT Blacklisted

I used cdbpreviously and it was working perfectly, except for networks (5.6.7). I switched to cidr which is apparently recommended, but I still get spam although the network is in my blacklist file.

I did the sudo postmap /etc/postfix/blacklist && sudo postfix reload after adding the network to the blacklist file, but it didn’t filter the spam mail.

Specify explicit network mask.

The syntax for the cidr table is different from simpler indexed files (such as DB or DBM), it expects an explicit mask - and uses it to allow for much nicer representation of blocks smaller or bigger than /8 /16 /24.

# network_address/network_mask result
192.0.2.0/24     REJECT Blacklisted
203.0.113.128/28 REJECT Blacklisted

Test your file using postmap -q:

# postmap -q 203.0.113.144 cidr:/path/to/example.cidr
# postmap -q 203.0.113.143 cidr:/path/to/example.cidr
REJECT Blacklisted

A warning will be emitted for syntax problems, no result will be printed for IPs not matching any listed network.