Rmail operates by default on your primary Rmail file, which is named ~/RMAIL and receives your incoming mail from your system inbox file. But you can also have other Rmail files and edit them with Rmail. These files can receive mail through their own inboxes, or you can move messages into them with explicit Rmail commands (Section 29.7).
Read file into Emacs and run Rmail on it (rmail-input).
Specify inbox file names for current Rmail file to get mail from.
Merge new mail from current Rmail file's inboxes (rmail-get-new-mail).
Merge new mail from inbox file file.
To run Rmail on a file other than your primary Rmail file, you may use the i (rmail-input) command in Rmail. This visits the file in Rmail mode. You can use M-x rmail-input even when not in Rmail.
The file you read with i should normally be a valid Rmail file. If it is not, Rmail tries to decompose it into a stream of messages in various known formats. If it succeeds, it converts the whole file to an Rmail file. If you specify a file name that doesn't exist, i initializes a new buffer for creating a new Rmail file.
You can also select an Rmail file from a menu. Choose first the menu bar Classify item, then from the Classify menu choose the Input Rmail File item; then choose the Rmail file you want. 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). These variables also apply to choosing a file for output (Section 29.7).
Each Rmail file can contain a list of inbox file names; you can specify this list with M-x set-rmail-inbox-list RET files RET. The argument can contain any number of file names, separated by commas. It can also be empty, which specifies that this file should have no inboxes. Once a list of inboxes is specified, the Rmail file remembers it permanently until you specify a different list.
As a special exception, if your primary Rmail file does not specify any inbox files, it uses your standard system inbox.
The g command (rmail-get-new-mail) merges mail into the current Rmail file from its specified inboxes. If the Rmail file has no inboxes, g does nothing. The command M-x rmail also merges new mail into your primary Rmail file.
To merge mail from a file that is not the usual inbox, give the g key a numeric argument, as in C-u g. Then it reads a file name and merges mail from that file. The inbox file is not deleted or changed in any way when g with an argument is used. This is, therefore, a general way of merging one file of messages into another.