When setting up Postfix to accept mail from other machines in your network, the primary configuration file you'll work with is /etc/postfix/main.cf. The key parameter here is mynetworks, which defines which IP addresses are allowed to relay mail through your server.

First, edit your main.cf file:

# nano /etc/postfix/main.cf

Add or modify these parameters:

inet_interfaces = all
mynetworks = 127.0.0.0/8, 192.168.1.0/24
relayhost = 
smtpd_relay_restrictions = permit_mynetworks, reject_unauth_destination

Replace 192.168.1.0/24 with your actual network subnet.

Before making changes, verify your basic connectivity from the Windows server:

telnet your.postfix.server 25

If this fails, check these CentOS firewall settings:

# firewall-cmd --permanent --add-service=smtp
# firewall-cmd --reload

For more granular control, consider using SASL authentication:

smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination

Check your mail logs when troubleshooting:

# tail -f /var/log/maillog

Common errors include:

• Relay access denied - indicates mynetworks misconfiguration
• Connection refused - firewall or Postfix not listening on the interface

For production environments, consider these optimizations:

smtpd_client_connection_limit = 100
smtpd_client_message_rate_limit = 100
smtpd_client_recipient_rate_limit = 100

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First, verify basic network connectivity between your Windows server and CentOS Postfix host. Run these commands from the Windows command prompt:

telnet postfix_server_IP 25
ping postfix_server_IP

If these fail, resolve basic networking issues before proceeding with Postfix configuration.

Edit the main Postfix configuration file:

sudo vi /etc/postfix/main.cf

Key parameters to modify:

# Allow connections from your local network (adjust IP range)
mynetworks = 127.0.0.0/8, 192.168.1.0/24

# Set your mail domain
myorigin = $mydomain
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

# Enable SMTP authentication (optional but recommended)
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_local_domain = $myhostname

Ensure your firewall allows SMTP traffic:

sudo firewall-cmd --permanent --add-service=smtp
sudo firewall-cmd --reload

For CentOS systems with SELinux enabled:

sudo setsebool -P allow_postfix_local_write_mail_spool on
sudo semanage port -a -t smtp_port_t -p tcp 25

After applying changes, test with:

sudo postfix reload
sudo systemctl restart postfix

Now try sending test email from your Windows server using this PowerShell script:

$EmailFrom = "user@yourdomain.com"
$EmailTo = "recipient@example.com"
$Subject = "Postfix Relay Test"
$Body = "This is a test email sent via Postfix relay"
$SMTPServer = "your_postfix_server_IP"
$SMTPClient = New-Object Net.Mail.SmtpClient($SMTPServer, 25)
$SMTPClient.EnableSsl = $false
$SMTPClient.Send($EmailFrom, $EmailTo, $Subject, $Body)

Check Postfix logs for troubleshooting:

sudo tail -f /var/log/maillog

Look for successful relay attempts with status=sent or authentication failures.

For more secure setup, configure SASL authentication:

sudo yum install cyrus-sasl-plain
sudo vi /etc/sasl2/smtpd.conf

Add these contents:

pwcheck_method: auxprop
auxprop_plugin: sasldb
mech_list: PLAIN LOGIN

Create SASL users:

sudo saslpasswd2 -c -u postconf -h myhostname username
sudo chown postfix:postfix /etc/sasldb2