We're building a self-hosted service similar to Mandrill for bulk and transactional email on https://highrisehq.com. Mandrill was the only service that offered unlimited sub-accounts each with their own reputation and quota. We relied on that to offer bulk email to our customers, and since there’s no good alternative we need to build it in-house. In addition to sub-accounts & quota management, we needed open+click analytics, templates, and a fast RPC api. We also don’t want an email provider going dark on us again, so it’ll support sending from a pool of email providers. Mailgun is the one we’re using first (it’s been great so far) then likely SES. 
We’re going to open source it at some point. It’s written in Go with a Postgres backend so deployment is straight forward — should be able to get decent mileage off a single Heroku dyno. I look forward to sharing more soon!