Yes, you will edit the allowed_recipient_addresses field in the VizAlertsConfig workbook. Here's what ours looks like. We allow alerts to anyone at the tableau.com domain by default, but for certain users, I let them email out to Salesforce and Pagerduty:
Because this is a calc, you can base your exceptions on any fields in the data source--workbook Id, the user (subscriber), project, etc.
Thanks for the quick response! I'll try it out and report back.
I was able to get it working by adding the extra email domains to the parameter as soon in the screenshot. Thanks!