Download Squad writes:
1. If you don’t need to read it now, it shouldn’t be in your inbox.
2. If you’ve already responded to it, it shouldn’t be in your inbox.
3. If it comes from a known source (some person, retailer or mailing list that sends you mail more often than once every few months) it should be labeled automatically.
4. No one needs to look at their own inbox more than once an hour (and for many, once every 2-3 hours).
5. To borrow from the cult of GTD, re-factor constantly and mercilessly.