@michoel The email is being sent from a custom record using the standard email form, not sure how I can attach a userevent script to the email form 🤔

You attach it to the message record. Just make sure to check for type since Faxes are also message record in NS.

@rustyshackles But then how do i populate the sublist. Can't get recordObj.selectNewLine({sublistId: 'otherrecipientslist'}) to work. Also is there a way to get the ID of the custom record from which am sending the email?

What do you mean by sublist, if you are sending an email from the custom record via the UI, it will be attached by the system. No additional action required