GNU Mailutils

3.5.4.4 Modifying the Headers: ~h, ~t, ~c, ~b, ~s

To add new addresses to the list of message recipients, use ‘~t’ command, e.g.:

~t name1@domain.net name2

To add addresses to Cc or Bcc, use ‘~c’ or ‘~b’ escapes respectively.

To change the Subject header, use ‘~s’ escape, e.g.:

~s "Re: your message"

Finally, to edit all headers, type ‘~h’ escape. This will present you with the values of To, Cc, Bcc, and Subject headers allowing to edit them with normal text editing commands.