Location of OSX Lion Mail Server sent mail
Ok, I found my error. The sent messages folder is into the same directory of the inbox, under /Library/Server/Mail/Data/..., but it's hidden From Finder and ls as it is named with a . To start the folder name - .../.Sent Messages.

My mistake was to copy the contents of the backup folder to the newly created folder without checking the presence of hidden folders. My mistake is your answer if you fall into the same "trap".