This help article is for an old version of Direct Mail.
If you are sending email via your own email server, Direct Mail will prevent your computer from going to sleep while sending email (or while waiting for a scheduled delivery to begin).
If you are sending via e3 Delivery Service, the delivery is handled by our servers, and thus Direct Mail will not prevent your computer from going to sleep. It is not necessary for Direct Mail to be open or for your Mac to be powered on.