Rublon sends emails using Postmark.
Rublon sends emails from one of the Outbound SMTP Servers.
Each of the SMTP servers has its IP range. There is no guarantee which of the IP addresses will be used to send you an email message.
To ensure that emails from Rublon will not be marked as spam, add all of these IP addresses to your safe sender list in Office 365 or your email allowlist in Google Workspace.
Helpful Links
Postmark - Outbound SMTP Servers