Remove the default configuration module.
Switch from constants to variables in the Configuration module. This should keep Ruby from complaining when users override the defaults. Change the local config path from $HOME/.mailshears.conf to $HOME/.mailshears.conf.rb so that Ruby will actually load it.
Reorder the default config file to make more sense.
Load the default configuration file in lib/mailshears.rb. Allow the user to load his own configuration file, overriding the defaults.