SMTP Error: [451] 4.7.1 Service unavailable - try again later

Hi All
I am currently on Scaleway cloud.

My problem is SMTP Error
SMTP Error: [451] 4.7.1 Service unavailable - try again later

They said they did not block SMTP port. When I check through SSH, the port are opened.

But when I check using WordPress SMTP they said block by firewall.

I have no issue receiving emails.

For those of you have not used Scaleway, they assign an internal IP address and 1 public IP address for your cloud.

Could this be because the server IP is using NAT?
I use cwp panel rDNS Checker and the national IP can't resolve to any hostname because using internal IP.

O currently use another cheaper cloud provider for emails because of this issue. (Installed CWP Panel)

Anyone have a solution or workaround?

Note:
1.Server IP is NAT IP address.
2.Share IP is using Public IP Address.