Configure your mail with mail.app on Mountain Lion
I suppose that most of the space required by your mails is due to the attachments. In this case you can avoid to store tham.

Indeed, in Mail.app you can configure IMAP for keeping only the text for off-line viewing and not the attachments.


Go to Mail > Preferences
Go to the tab Accounts and select your account
Go to the sub-tab Advanced
In the first combo-box select "All messages, but omit attachments"