These commands copy messages from an Rmail file into another file.
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.
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.
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 Menu item; then choose the Rmail file you want.
This outputs the current message to that file, like the o command.
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
t; then the o and C-o commands delete the original
message after copying it. (You can undelete the original afterward if
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. See section Display of Messages.
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
(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.
applies to both o and C-o.