These commands copy messages from an Rmail file into another file.
Append a copy of the current message to the file file, using Rmail file format by default (rmail-output-to-rmail-file).
Append a copy of the current message to the file file, using system inbox file format by default (rmail-output).
Output just the message body to the file file, taking the default file name from the message Subject header.
The commands o and C-o copy the current message into a specified file. This file may be an Rmail file or it may be in system inbox format; the output commands ascertain the file's format and write the copied message in that format.
When copying a message to a file in Unix mail file format, these commands include whichever header fields are currently visible. Use the t command first, if you wish, to specify which headers to show (and copy).
The o and C-o commands differ in two ways: each has its own separate default file name, and each specifies a choice of format to use when the file does not already exist. The o command uses Rmail format when it creates a new file, while C-o uses system inbox format for a new file. The default file name for o is the file name used last with o, and the default file name for C-o is the file name used last with C-o.
If the output file is an Rmail file currently visited in an Emacs buffer, the output commands copy the message into that buffer. It is up to you to save the buffer eventually in its file.
Sometimes you may receive a message whose body holds the contents of a file. You can save the body to a file (excluding the message header) with the w command (rmail-output-body-to-file). Often these messages contain the intended file name in the Subject field, so the w command uses the Subject field as the default for the output file name. However, the file name is read using the minibuffer, so you can specify a different name if you wish.
You can also output a message to an Rmail file chosen with a menu. Choose first the menu bar Classify item, then from the Classify menu choose the Output Rmail File menu item; then choose the Rmail file you want. This outputs the current message to that file, like the o command. The variables rmail-secondary-file-directory and rmail-secondary-file-regexp specify which files to offer in the menu: the first variable says which directory to find them in; the second says which files in that directory to offer (all those that match the regular expression).
Copying a message gives the original copy of the message the filed attribute, so that filed appears in the mode line when such a message is current. If you like to keep just a single copy of every mail message, set the variable rmail-delete-after-output to t; then the o and C-o commands delete the original message after copying it. (You can undelete the original afterward if you wish.)
Copying messages into files in system inbox format uses the header fields that are displayed in Rmail at the time. Thus, if you use the t command to view the entire header and then copy the message, the entire header is copied. Section 29.13.
The variable rmail-output-file-alist lets you specify intelligent defaults for the output file, based on the contents of the current message. The value should be a list whose elements have this form:
(regexp . name-exp)
If there's a match for regexp in the current message, then the default file name for output is name-exp. If multiple elements match the message, the first matching element decides the default file name. The subexpression name-exp may be a string constant giving the file name to use, or more generally it may be any Lisp expression that returns a file name as a string. rmail-output-file-alist applies to both o and C-o.