ASK: Self-hosted GMail alternatives?

linkedlinked writes: "I'm tired of building my sandcastles on Google's beachfront. I've moved off Docs, Plus, and Analytics, so now it's time to host my own email servers. What are the best self-host open-source email solutions available? I'm looking for "the full stack" — including a GMail-competitive web GUI — and don't mind getting my hands dirty to set it up. I leverage "most" of GMail's features, including multi-domain support and fetching from remote POP/IMAP servers. Bonus points: Since I'm a hobbyist, not a sysadmin, and I normally outsource my mail servers, what new security considerations do I need to make in managing these services?"