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How to Fix Postfix Accepting Email from the Wrong Domain
If your Postfix mail server is accepting emails from incorrect or unauthorized domains, it can pose serious security risks, including spam, phishing attacks, or unauthorized access. Here’s how to address this issue:
1. Verify Postfix Configuration:
- Check your
main.cf
andmaster.cf
configuration files to ensure that your server is configured to accept emails only from authorized domains.
2. Implement Restrictions on SMTP:
- Postfix allows you to set up restrictions on who can send mail to your server. These restrictions are usually defined in the
smtpd_sender_restrictions
,smtpd_recipient_restrictions
, andsmtpd_client_restrictions
parameters. Example configuration:
smtpd_sender_restrictions = reject_unknown_sender_domain,
reject_non_fqdn_sender,
permit_mynetworks,
reject_unauth_pipelining,
reject_unauth_destination,
check_sender_access hash:/etc/postfix/sender_access
smtpd_recipient_restrictions = permit_mynetworks,
reject_unauth_destination,
check_policy_service unix:private/policyd-spf
smtpd_client_restrictions = permit_mynetworks,
reject_rbl_client zen.spamhaus.org
In this configuration:
permit_mynetworks
allows hosts in your trusted networks to send mail.reject_unknown_sender_domain
andreject_non_fqdn_sender
block emails from domains that are not fully qualified or unknown.reject_unauth_destination
blocks attempts to send emails to destinations outside your server’s authorized domains.- The
check_sender_access
option references a file (/etc/postfix/sender_access
) that lists permitted or denied senders.
3. Edit the Sender Access File:
- Create or update
/etc/postfix/sender_access
to specify which domains or email addresses are allowed or blocked. Example entries:
example.com OK
badexample.com REJECT
.untrusteddomain.com REJECT
- After editing, generate the database file with:
bash postmap /etc/postfix/sender_access
- Reload Postfix:
bash systemctl reload postfix
4. DNS and SPF Configuration:
- Ensure your server’s DNS records (A, MX) are correctly set up.
- Implement Sender Policy Framework (SPF) records for domain authentication to prevent spoofing.
- Use the
check_policy_service
for SPF validation.
5. Secure Your SMTP Relays:
- Limit SMTP relay to authenticated users and trusted networks.
- Configure
mynetworks
parameter to include only trusted IP addresses. Example:
mynetworks = 192.168.1.0/24, 127.0.0.0/8
6. Enable Logging and Monitor:
- Increase logging verbosity in Postfix to monitor unauthorized access attempts. Use the
maillog
orsyslog
files for this purpose. - Regularly check logs for suspicious activity.
7. Use DNSBL (DNS-based Blackhole List):
- Implement DNSBLs to block emails from known spam sources. Example:
smtpd_recipient_restrictions = reject_rbl_client zen.spamhaus.org
8. TLS/SSL Configuration:
- Ensure all communications are encrypted. Configure Postfix to enforce TLS/SSL. Example:
smtpd_tls_cert_file = /etc/ssl/certs/yourdomain-cert.pem
smtpd_tls_key_file = /etc/ssl/private/yourdomain-key.pem
smtpd_tls_security_level = encrypt
smtpd_tls_auth_only = yes
9. Check for Open Relays:
- Test if your server is an open relay using tools like
telnet
or online open relay checkers. Close any relay settings that allow unauthorized email relaying.
10. Regular Updates and Security Audits:
- Keep your Postfix installation and underlying OS up to date with the latest security patches.
- Conduct regular security audits to ensure no configurations allow unauthorized email acceptance.
By carefully configuring your Postfix server following these steps, you can significantly reduce the risk of accepting emails from unauthorized or incorrect domains.