To temporarily block inbound mail to a mail server by blocking access to port 25 on the server use the following iptables command (this allows you the mail server process to continue to run so you can sort resolve any problems before allowing access):
iptables -A INPUT -p tcp -d <SERVER_IP> --dport 25 -j REJECT
This assumes your input rule is called INPUT, if you have existing custom chains/rules you may need to update this command to fit your configuration.
To allow mail back into the server delete the rule using:
iptables -D INPUT -p tcp -d 188.8.131.52 --dport 25 -j REJECT
You can view existing rules using
iptables -L -nTags: block, email, iptables, linux