You can't provide a merge field in Additional Emails; it has to be actual email addresses.

CustomComponent is correct solution for this but how do I close this? Should I create my own answer?

@cropredy Custom component seems to be the way; I was checking if there was a way to avoid it.

@TheSalesforceGeneral Thanks. In my case, its a visualforce email template so I dont think Javascript would work.

Heres what we have found: Constants.pm doesn't list all the custom objects that get_tables() show. For e.g., we have a custom object Daily_Usage__c which is not constants.pm but the query on that works fine. ContractAsset__c is not in their but the query on it doesn't work.

Regarding get_tables(), it was an issue on our side and we are getting all the object names in get_tables() EXCEPT for this object :(

I am checking with my integration team on this. Will update soon.

I will check on the part about Constants.pm. Regarding the API not having access to the object, I logged in as the API user in Salesforce and was able to view a record of that object. There is also no namespace that we have defined.

Thank you. I was able to replicate the issue in sandbox based on this info.