Good write up. I had a distantly similar problem last year. Needed a poor man's transactional email and newsletter blaster. I wanted to leverage my ruby app code and make calls to a mail delivery engine. Problem is, things fail, and you don't want to send the same email under the same conditions twice. I ended up with a brute force failsafe by logging a "signature" of each message send to a mongodb collection and do a look up as a safety check prior to each send (or batch). Its not efficient, but works and when my mailer fails, which it does often, I simply restart it and don't worry about ticking off my user base. I end up with a rather large mongodb collection, but every few months, I prune it by removing log items that I know won't happen again.