rmail-pop-password

rmail-pop-password is a variable defined in `rmail.el'.
Its value is

nil

• This variable is an alias for `rmail-remote-password'.
• This variable is obsolete since 22.1;
  use `rmail-remote-password' instead.

Documentation:
Password to use when reading mail from a remote server.
This setting is ignored for mailboxes whose URL already contains a password.

You can customize this variable.