where number is the number of the letter to be inserted. For example, to send a letter to another user that includes a copy of letter number 3 from your mailbox list, you would do the following:
On a new line give the command ~m 3 and then press Return.
mailx displays the message, Interpolating: 3 (continue)
You won't see the text of message 3, but the recipient will. You can go on with your letter after (continue), or you can send it as is.