I agree that e-mail offers some attractive features in terms of queueing messages and handling temporary delivery failures robustly, and I have heard people talk about doing it this way before. I would say it mostly just solves the message queue issue though.

It might have issues with frequent polling, just like a database-backed queue would, although I suppose an e-mail server is built to do that well. It would also be difficult to get the current status of a job, but maybe "received" is good enough for most cases.

I think you would end up needing to build some extra infrastructure to allow a client to check for the response to a specific job, or is there a simple way to do that with IMAP queries and subject lines or something? Also, how would you handle distributing messages across a set of worker processes? Would you use e-mail for that too, sending messages on from a dispatcher to specific workers with unique e-mail addresses?