s number filename
where number is the number of the letter to be saved and filename is the file where you want to save the letter. For example, to save letter 3 into a file called ~/notes/finance, you would type:
& s 3 ~/notes/finance
(Remember that in a path name, the ~ represents your home directory.)
You can also save several letters at once to the same file. For example, to save letters 3, 5, 6, 7, and 8 to ~/notes/finance, you would type:
& s 3 5-8 ~/notes/finance
If the file you specify does not exist, mailx creates it. If the file does exist, mailx appends the letter you are saving to the end of the file.