It is possible to publish the action for an invalid text in the email step "To" field.
Steps to Reproduce
Create a new action.
Add an Email Step.
Enter any text like "abcd.jhgfgt" in "To" field and subject.
This is accepted.
Publish the action.
Action is published successfully.
Add this action to a flow.
Trigger the workflow.
The flow execution gives a runtime error for the email step, and invalid text is accepted for email "To" field. No validation occurs similar to when inserting a wrong email on the incident form watch-list.
Ensure that you add valid email addresses to the Email Step actions.
No feedback about invalid email step recipient values occurs until runtime. At runtime, the email API error surfaces in the flow execution report. This sort of general email address validation is provided on standard glide forms via the native email field type (not present in the Flow designer UI currently).
Related Problem: PRB1248600