Perhaps I'm not seeing the forest for all the trees here but what happens if you have 10 people to send to, half way through an exception is raised (e.g. network error with ExactTarget's server) and 5 people still haven't received their email. 
When the backfiller sees this it will reattempt with the same `run_datetime` later. 

I don't think that's unrealistic at all. Networks are always flaky. 

Do we need to run `connection.commit()` after each `update_user()`?