You use the same commands to save or copy letters into folders as into files, except that the folder name is preceded by a plus sign (+) instead of a path name. The + tells mailx that the folder is to be kept in the folder directory (Letters).
For example, to save letter 3 to a folder called projects, type:
& s 3 +projects
mailx interprets this command as meaning "save letter 3 into ~/Letters/projects." (If the folder doesn't already exist, mailx will create it.)
Copy the letter into a folder by typing: