Sending email messages at a scheduled later time was supposed to be a feature built into Thunderbird 2.0 but wasn’t. Thereis a great plug-in, though, at http://www.unsignedbyte.com/?page_id=4:
Send Later Extension [SL8TR] sl8tr.xpi
Install it, and then compose a message, and then look in FILE, SEND LATER.
I will use this for my weblog.
To see what you’ve got scheduled, go to the server’s DRAFTS folder, not the local one. The time given is the time of writing, not the time of sending. To see the time of sending, go to VIEW and HEADERS and click so you can see ALL the headers for each message, not just the normal ones. The time-of-sending info is in the header.