What happens if nothing is entered in a custom field when a Custom Field Tag is used in an email?

When a Custom Field Tag is utilized in an email and no value is entered in the corresponding custom field, the result is that the tag will not insert anything into the email when it is sent. This behavior is designed to maintain the integrity of the email content, ensuring that placeholders without values do not lead to confusion or an unprofessional appearance. As a result, those fields will simply be absent from the final email, rather than displaying an empty value or a placeholder which could detract from the overall clarity and effectiveness of the communication.

Other options suggest that something is inserted in some form (like the custom field name or a default placeholder), or imply that the email cannot be sent, which is not the case. Therefore, the correct answer highlights the intended functionality of the custom fields in a way that helps preserve the quality of the outgoing messages.