Open the Terminal and run the following command:
defaults write com.apple.Finder AppleShowAllFiles true
Then open the Time Machine disk and navigate through the backups to the various instances of your user home folder and see if the "Library" folder is listed there.
The Postbox data folder is located here:
~[your home folder]/Library/Application Support/PostboxApp
To undo the show all files command, just repeat the above command with "false" at the end of it instead of "true."