Thunderbird has an Outbox folder in the Local Folders account, but Thunderbird usually sends messages directly without using the Outbox. Thunderbird uses the Outbox when it is offline. You can also use the Outbox when Thunderbird is online.
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 servers normally. When it is in its offline state, it cannot 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
You can use the Outbox to store messages that you write, so that they are not sent until you choose to send them.
If Thunderbird is offline, then the Send button in the Write window changes to Send Later, which stores the message in the Outbox to be sent later. If Thunderbird is online, then you can still use the Outbox by choosing File – Send Later (or by pressing Ctrl+Shift+Return).
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. This only works well if Thunderbird is running at the time the messages are to be sent. Otherwise, the messages will not be sent until Thunderbird next starts up. Here are some examples of extensions that might be suitable.
The Blunder Delay extension sends all the messages in the Outbox together at scheduled times. It works with Thunderbird 3 and earlier versions.
The Mail Tweak extension has a Defer tweak for scheduling messages to be sent at at individual times. It works in Thunderbird 3, and it can also defer received messages. This extension stores deferred messages in a Deferred folder that is a subfolder of the Outbox. When it is time for an individual deferred message to be sent, the extension moves the message to the Outbox and sends all the messages in the Outbox. The Collected reply tweak also uses the Outbox.
The SendTools extension and the Send Later extension are for scheduling messages to be sent at at individual times in older versions of Thunderbird. Send Later 3 is a version of the Send Later extension that has been updated to work with Thunderbird 3.
Note: The Outbox extension is obsolete. It does not support uninstalling and so it is not recommended.
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:
Thunderbird's Outbox may be developed in future. Sending messages in the background, and automatic retries on errors are work in progress. See this bug report, Thunderbird_3.0_-_New_Features_and_Changes#Send_in_Background and this wiki article for more information.