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You have the following option with Mac's Mail:
Export your inbox to a .mbx file and give it an appropriate name, e.g. inbox Aug2008.mbx
Same can be done with the sent folder
You can re-import the .mbx file to Mail for viewing.
There are some mail mangagement progs available commercially (mailsteward is one I've googled, don't know its details/usefulness though as don't intend to go down 3rd party route)
...hope this helps