Using the "Send Later" command
The Send Later command will enable you to compose messages and then send them at a later point in time. This is particularly useful when you know you'll be going offline, for example, when traveling by plane.
When you're finished composing a message in the Compose window, go to File > Send Later. This will move the message to the Outbox folder, located in your Local Folders (Windows) or On My Mac (macOS) account. You can save as many messages as you like to the Outbox folder.
When you go back online again, Postbox will offer to send messages within the Outbox.
To manually send messages within the Outbox folder, select Send Unsent Messages from the File menu.