Hey Barnes, here the top 10 tips to Secure Your Email Server...
- Configure mail relay options carefully to avoid being an Open Relay
- Set up SMTP authentication to control user access
- Limit connections to protect your server against DoS attacks
- Activate Reverse DNS to block bogus senders
- Use DNSBL servers to fight incoming email abuse
- Activate SPF to prevent spoofed sources
- Enable SURBL to verify message content
- Maintain local IP blacklists to block spammers
- Encrypt POP3 and IMAP authentication for privacy concerns
- Have at least 2 MX records for failover
Hope this will helps you 🙂