Hi @Dhathrig: I assume you are using the Notify option or the send email utility to send emails. In such case, you will have a option to select a corr rule where you can refer the rejection reason property.

Please refer: https://academy.pega.com/topic/email-correspondence/v4

For approval rejection smart shape, it has the option to edit the corr. Click on the configure icon next to email content dropdown ( next dropdown if use existing is selected) to update the corr.

Thanks