In the When to Send drop down, there are up to 3 options.
If you chose “Now, it is urgent”, the email will be sent when you click “Create”.
If you chose “Not Yet, I am still editing it”, it will save the email as a draft in the email list in which it was created. The “Sent at” column will show “pending”.
When you are ready to send the email, click the pencil icon to edit it, and chose “Now, it is urgent”. Since you had previously saved the email, the button will say “Update” instead of “Create”. Clicking “Update” will send the email. You can edit and update the email as frequently as you like while the “When to send” continues to be set to “Not Yet, I am still Editing it”. It will only be sent when you change the setting to “Now, it is urgent” and click the “Update” button to save it.
The third option is to place the email in the sunday newsletter. This is only available if you have the Sunday Newsletter turned on. If the email has been saved with this option, when the Sunday Newsletter is sent, it will include the email in the newsletter, and mark the email as having been sent.