I have 14GB of email (close to 1 million messages) on my Mac that I want to take up less space while still being able to reference "archived" messages. I have a set of folders that are effectively read-only, and I only need to access them once in a while. I'm using Apple Mail.
A subgoal is to speed up my online backup software, which is backing up each message individually, taking a long time.
I tried creating a compressed DMG for one of the folders I want to archive and creating a symlink to the mounted DMG from within the ~/Library/Mail directory. This almost worked, except that, when I unmounted the DMG, Apple Mail eventually "forgot" about the messages, and when I re-mounted the DMG (and restarted Apple Mail), the folders appeared with no messages in them. The 'Rebuild' option was grayed out, presumably because the compressed DMG can only be mounted as read-only. So there was no way to see any of the messages in the archive.
Are there any other ways to do this using Apple Mail. (I would consider another email program only as a last resort; Thunderbird repeatedly crashes on me, and I don't like Microsoft Outlook.)