Mail Server Guides

Sending mail between trusted mail servers

When two privately operated mail servers communicate directly, both servers should have a valid public identity and reverse DNS.

Server B — mail.typist.kr

Server B uses:

mail.typist.kr
119.201.166.00

Its Postfix configuration identifies the server itself as a trusted local network:

sudo postconf mynetworks

mynetworks = 127.0.0.0/8, 119.201.166.00/32

The individual server can also be placed in the trusted-client access table when an explicit server-to-server exception is needed:

sudo nano /etc/postfix/trusted_clients

119.201.166.00    OK

After modifying the access table:

sudo postmap /etc/postfix/trusted_clients
sudo postfix check
sudo systemctl reload postfix

Server A — phd.co.kr

The other mail server is:

mail.phd.co.kr
169.213.166.00

Its trusted network configuration contains:

sudo postconf mynetworks

mynetworks = 127.0.0.0/8, 169.213.166.00/32

The trusted-client database is updated with:

sudo postmap /etc/postfix/trusted_clients
sudo postfix check
sudo systemctl reload postfix

Why reverse DNS matters

Mail servers commonly perform reverse-DNS checks on incoming SMTP connections. A server connecting from an IP address without a usable PTR record may be treated as suspicious or rejected.

For mail.typist.kr, the public reverse DNS relationship is:

119.201.166.00
        ↓ PTR
mail.typist.kr

Forward and reverse DNS should therefore agree:

mail.typist.kr
        ↓ A
119.201.166.00

119.201.166.00
        ↓ PTR
mail.typist.kr

Why use /32?

A /32 IPv4 network represents exactly one address. Therefore:

119.201.166.00/32

means that only 119.201.166.00 is trusted. It does not authorize the surrounding KT/KORNET address space.

Checking the configuration

sudo postconf mynetworks
sudo postconf -n | grep -Ei 'smtpd.*restriction|reject.*hostname'
sudo postfix check

When troubleshooting mail delivery, the Postfix log is also useful:

sudo tail -f /var/log/mail.log