The relevance for the Automatic group is evaluated locally on the client. If I understand you correctly, you want to call the REST API to create an automatic group. The relevance defining the constraints of the automatic group would be populated from an external list of email addresses?
What local property are you evaluating the email address against?
Creating the initial automatic group would be fairly straightforward. You could use PowerShell to modify the xml document with your data source values, before hitting the REST API to submit the request.
How often is the source data modified after that?
Would be also interesting to know about the overall use case, in case there’s a way to use session relevance to achieve what you need without having to create a group.
Data mapping could also be an option - where you match external source data to client properties so you can use the data points for groups, targeting, reporting etc.