Any more info you can provide regarding your use case would be helpful. Are the attributes completely unknown ahead of time? Do you have a number of attributes and you might only use a subset of those?
*** EDIT ***
you could use a data structure like this, which is what I think you were describing
The difficult thing would be to create the outgoing JSON, unless your service has a generic structure for sending information to be used to send an email. If it does not, you would probably have to create a JSON string ‘by hand’ in a microflow and include that as the outgoing body of the REST call. If the email send service has a generic structure, you could import that JSON into Mendix, create an Export Mapping and then populate a set of entities in a microflow to be used by the Export Mapping when you call the REST service.
If you have specific info about the Send Email service, I would be interested to take a look at that and see if I can be more specific.