Because of private server was not registered with major anti-spam organizations, SMTP cannot send inbox to popular email providers like: Gmail, AOL, Outlook, Yahoo

Standard of SMTP

Test Deliver: 

Title & Messenger ( Plain Text ): " BEST GOOD FOR YOU. THANKS ORDER MY SERVICE "

Delivery Rate:

+ Yahoo: Spam

+ AOL: Spam

+ Domain Mail ( No firewall settings & Mail filtering system): Inbox

This mail server don't have a system to filter mail, fight spam, Block ads, Block phishing. It was created from the enterprise server ( Buy Server & Install )

Mail services without firewalls and mail filtering system:

https://mail.naver.com 

http://zoho.com

https://mail.yandex.com

China Domain mail.

... Etc

+ Domain Mail ( There are firewalls and mail filtering systems ): Not Deliver

Mail service has firewall and mail filtering system:

OFFICE365 - https://outlook.office365.com

WEBMAIL Godaddy - http://godaddy.com

Google Suite: https://gsuite.google.com

... Etc