Unable to receive/send e-mails.
It is not possible to connect to the server by telnet on port 25:
# telnet <server_ip> 25 Trying 10.10.10.10... telnet: connect to address 10.10.10.10: Connection refused
SMTP Port 25 is blocked by firewall.
Add the following rule to the firewall:
iptables -I INPUT 3 -p tcp -m state --state NEW -m tcp --dport 25 -j ACCEPT
If the firewall is configured in Plesk, open 25 port on the server in Tools & Settings-> Firewall