Symptoms
I would like to use a service parameter or an account attribute as a placeholder in the notification template. It works correctly in the template body, but attempts to use it as an recipient address ("To" field) fails with a validation error:
Email address is invalid. Please correct.
Cause
There is an internal validation of To/From addresses. This fields can only contain either the real e-mail addresses or placeholders with names with Email at the end (e.g. @@Cust_Account_AdminContact_Email@, @@Cust_Account_BillingContact_Email@, etc).
Resolution
Use appropriate parameters/attributes names (must end with Email) if you intend to use them as addresses in the notification templates.
There is a related request with id #PBA-78261 ("Usage of custom attribute as an email address placeholder") to improve the email validation.