Send Later is a Thunderbird add-on that allows you to write an email message and define when you want it to be sent. The message is saved into your Drafts folder, and is delivered at approximately your specified time.

In Thunderbird, open the add-ons dialog with the Tools > Add-ons menu command. Click the “Get Add-ons” button. Enter “send later” in the search box and hit Enter. When the “Send Later” add-on shows up in the search results, click the “Add to Thunderbird…” or “Install” button. When the “Software installation” dialog pops up, click the “Install Now” button when it becomes active. Restart Thunderbird after the installation.

When you want to schedule a message for later delivery, either select the File > Send Later menu command in the message composition window, or hit Ctrl-Shift-Enter.

Specify a specific time at which to send the message. Tell Send Later in the text box when you would like the message to be sent, or use the date and time pickers below the text box if you prefer. The text box understands lots of different formats, so give it a try!