Normally, the Unsent Messages is stored in "Unsent Messages"
(you have to change mail.default_sendlater_uri entry from mailbox://nobody@Local%20Folders/Unsent%20Messages to your mailbox since there is no individual mailbox setting for that)
For reason unknown, it becomes Unsent in the T400. In the filesystem, the mailbox under profile is still "Unsent Messages", just the display name is wrong. Not sure how you can change that.