An “hidden” feature in Thunderbird, makes the email client queue the message and send it in the background, one of the neat Outlook functionalities I’ve been missing since I switched to Thunderbird years ago.

The “send in background” capability is not exposed in the preferences (at least as of TB 3.1.1) — to enable the it, go into Preferences -> Advanced -> General -> Config Editor, change mailnews.sendInBackground to true and restart Thunderbird.

According to the Mozilla Wiki:

The following parts are implemented:

• Basic send in background.
• Sending a message will temporarily (for about a second) put it in the Outbox in Local Folders, and then Thunderbird will attempt to send it.
• Activity Manager progress and notifications
• The activity manager will echo progress and notifications of sending messages in the background (or via Send Later).
• Partial shut down support.
• Sending a message and attempting to quit should bring up a dialog which is retained until the message is sent. On Windows and Linux this will only happen if File -> Quit is used not the ‘x’ on the window.

Not implemented:

• If there is an error, Thunderbird won’t attempt to automatically send the message a second time (until another message is sent).
• If there are multiple messages queued, Thunderbird won’t attempt to automatically send other messages that are queued.
• Thunderbird won’t notify the user if messages are being sent during shutdown.