Domain Mail (No firewall settings & Mail filtering system) is created from a private server.
Almost all free email services ( Google, Outlook, Office365, Yahoo, AOL ...etc ) have firewalls & Mail filtering system.
A strong or weak firewall system depends on the size of the business. If it's a big company. They invest a lot of money building firewalls & Mail filtering system.
But if it's a small company. Firewall system investment is not required.
List of free or paid email services with firewalls & Mail filtering system
+ Google: very strong
+ Outlook / Office365: : very strong
+ Mail.ru: strong
+ Godaddy: strong
+ Yahoo: medium
+ AOL: medium
+ CPanel install on private server ( Hosting Services ): medium
+ 126.com: medium
+ 163.com: medium
+ Naver.com: weak
+ Yandex.com: weak
+ Zoho.com: weak
Domain Email has many types. You should filter Domain Mail (No firewall settings & Mail filtering system) in your domain mail list.
The following is a list of all mail domains sorted:
In the sorted list: Others ( ...... ): This is Domain Mail (No firewall settings & Mail filtering system)