Thanks everyone for all the good advice! After religiously backing up my entire mail folder, I compacted my mail database. The entire process took about 15 minutes, much less than I had feared, and trimmed the database size from 1.9 GB to 481 MB. Yay! This gives me more breathing room before I'm forced to export older messages to a separate db.
-- marco -- It's not the data universe only, it's human conversation. They want to turn it into a one-way flow that they have entirely monetized. I look at the collective human mind as a kind of ecosystem. They want to clear cut it. They want to go into the rainforest of human thought and mow the thing down.