From MozillaZine Knowledge Base
Thunderbird has an Outbox folder in the Local Folders account for deferred or delayed sending, for example when Thunderbird is offline. (Normally, Thunderbird usually sends messages directly without using the Outbox.)
Note: In previous versions of Thunderbird, the Outbox was named Unsent Messages.
Online and offline states
When Thunderbird is in its online state, it can connect to and interact with servers normally. In its offline state, Thunderbird will not connect to any servers, which means it cannot send mail, receive mail or check for mail.
To switch Thunderbird between its online and offline states, choose File –> Offline –> Work Offline. If the option is checked, Thunderbird is offline. Alternatively, click the icon at the left of the status bar at the bottom of Thunderbird's main window. If you cannot see any status bar there, choose: View – Status Bar If the Icon has a red marker with an X in it, Thunderbird is offline.
To configure some settings relating to online and offline states, choose Tools – Options –> Advanced –> Network & Disk Space, and press the Offline… button. You can configure Thunderbird's state when it starts up, how Thunderbird alerts you to the presence of unsent messages in your Outbox, and whether Thunderbird ensures that all available messages are downloaded before going offline.
To configure Thunderbird always to start up offline, use the Config Editor to set the preference offline.startup_state to 3.
Using the Outbox
The Outbox stores messages that you have written, for sending at a later time.
If Thunderbird is online, you can defer sending of a message by choosing File –> Send Later (or by pressing Ctrl+Shift+Return).
If Thunderbird is offline, then the Send button in the Write window changes to Send Later, which stores the message in the Outbox.
To make a Send Later button available on the toolbar even when Thunderbird is online, install an extension that provides such a button — for example, Magic SLR. After installing the extension and restarting Thunderbird, customize the toolbar to add the button.
You can work with messages in the Outbox in the the same way as messages in any other folder. For example, you can delete a message there, or edit it as new to create a new message.
Sending mail in the Outbox
To send all the mail in the Outbox, choose: File –> Send Unsent Messages Alternatively, get a context menu for the Outbox (for example, by right-clicking it) and choose: Send Unsent Messages
You cannot select which messages in the Outbox will be sent. You can only send them all. If you want to send some messages in the Outbox but not others, you could move the others to another folder temporarily.
When Thunderbird is offline and you switch it online, and there are unsent messages in the Outbox, Thunderbird normally asks you whether you want to send them. You can clear the checkbox "Always ask me when I go online" in the alert to prevent it from appearing again. You can reset the checkbox by using the Offline Settings dialog described above.
To send messages from Thunderbird at scheduled times, you must install an extension. Thunderbird must be running at the time the messages are to be sent, otherwise messages will not be sent until Thunderbird next starts up. Some possibilties:
Other ways to schedule sending
If you want to schedule messages to be sent while Thunderbird is not running, then you must register to use an Internet service. You can still write messages in Thunderbird when you use one of these services, but you send the messages to a special e-mail address. The service stores the messages and re-sends them at the time you specify. Here are some examples:
Stop sending a message
Thunderbird doesn't support any way to stop sending a message. Doing that by killing the process can be dangerous as Thunderbird keeps a number of files open that it periodically writes to. The best solution would be to use an add-on like BlunderDelay that saves the message to the Outbox folder, and sends it several minutes later on. You could either delete the message in the Outbox folder or move it to the Drafts folder, edit it, and send it again.
Another possibility would be to quickly switch to working offline by using the mouse to toggle the working online/offline icon in the lower left corner of the main window. This is very timing dependent, you need a large enough message that you have time to switch to working offline before it finishes sending the message. If successful you will get a "Sending of message failed. Please verify that your Mail & Newsgroups account settings are correct and try again" popup error.
Thunderbird's Outbox may be enhanced in the future. For example, sending messages in the background, and automatic retries on errors are work in progress. See Thunderbird_3.0_-_New_Features_and_Changes#Send_in_Background for more information.