When getting new mail, Rmail first copies the new mail from the inbox file to the Rmail file; then it saves the Rmail file; then it truncates the inbox file. This way, a system crash may cause duplication of mail between the inbox and the Rmail file, but cannot lose mail. If rmail-preserve-inbox is non-nil, then Rmail will copy new mail from the inbox file to the Rmail file without truncating the inbox file. You may wish to set this, for example, on a portable computer you use to check your mail via POP while traveling, so that your mail will remain on the server and you can save it later on your workstation.
In some cases, Rmail copies the new mail from the inbox file indirectly. First it runs the movemail program to move the mail from the inbox to an intermediate file called ~/.newmail-inboxname. Then Rmail merges the new mail from that file, saves the Rmail file, and only then deletes the intermediate file. If there is a crash at the wrong time, this file continues to exist, and Rmail will use it again the next time it gets new mail from that inbox.
If Rmail is unable to convert the data in ~/.newmail-inboxname into Babyl format, it renames the file to ~/RMAILOSE.n (n is an integer chosen to make the name unique) so that Rmail will not have trouble with the data again. You should look at the file, find whatever message confuses Rmail (probably one that includes the control-underscore character, octal code 037), and delete it. Then you can use 1 g to get new mail from the corrected file.
Some sites use a method called POP for accessing users' inbox data instead of storing the data in inbox files. movemail can work with POP if you compile it with the macro MAIL_USE_POP defined. (You can achieve that by specifying -with-pop when you run configure during the installation of Emacs.) movemail only works with POP3, not with older versions of POP.
Assuming you have compiled and installed movemail appropriately, you can specify a POP inbox by using a "file name" of the form po:username, in the inbox list of an Rmail file. movemail handles such a name by opening a connection to the POP server. The MAILHOST environment variable specifies the machine to look for the server on; alternatively, you can specify the POP server host name as part of the mailbox name using the syntax po:username:hostname.
Accessing mail via POP may require a password. If the variable rmail-pop-password is non-nil, it specifies the password to use for POP. Alternatively, if rmail-pop-password-required is non-nil, then Rmail asks you for the password to use.
If you need to pass additional command-line flags to movemail, set the variable rmail-movemail-flags a list of the flags you wish to use. Do not use this variable to pass the -p flag to preserve your inbox contents; use rmail-preserve-inbox instead.
The movemail program installed at your site may support Kerberos authentication. If it is supported, it is used by default whenever you attempt to retrieve POP mail when rmail-pop-password and rmail-pop-password-required are unset.
Some POP servers store messages in reverse order. If your server does this, and you would rather read your mail in the order in which it was received, you can tell movemail to reverse the order of downloaded messages by adding the -r flag to rmail-movemail-flags.