GYB for Mac
GYB (Got Your Back) is a command line tool written in Python that allows you to back uo your Gmail messages to your local computer.
GYB uses the standard IMAP protocol and takes advantage of various custom Gmail IMAP extensions.
GYB is cross-platform and it works on Mac OS X, Windows and Linux. Binaries for the Windows and Linux platforms are available on the project's homepage.
Detailed instructions on how to install and use the GYB utility on your Mac are available HERE.