Emails can be marked as transactional.
In the email builder, select Email Settings tab and click the checkbox for Transactional Email.
Once the Transactional Email checkbox is selected, the email will be considered as transactional and will be sent as such.
NOTE: As transactional emails do not contain %UNSUBSCRIBE% they cannot be sent as marketing / promotional emails as they might be reported as spam by the end recipients. Please make sure you differentiate between emails that are for marketing and those that are transactional.
If your email contains %UNSUBSCRIBE%, but is marked as transactional, then you will get the following error, prompting you to remove the %UNSUBSCRIBE%.