Installation Guide:

1. How to Setup Postfix/DKIM/SPF on Ubuntu with or without Plesk Onyx
2. How to setup DKIM/SPF/DMARC in Ubuntu with Plesk Onyx
3. How to setup SPF (Sender Policy Framework)
4. How to setup DMARC and Benefits
5. Generate a public and private key from OpenSSL (if require)
6. How to install/setup dkim in cPanel
7. Server Speedtest from command
8. User unknown in virtual alias table

Important Tools

For DKIM Checker: Read more