> A user will always end up in the same bucket. This is to ensure that a single machine can send out an e-mail that contains *ALL* the relevant activities for that user.

I'm not sure how the user consistently arriving in the same user collection bucket is the thing that ensures they get all relevant emails. Are you sure this is explained properly? If the fact that the user always ends up in the same bucket is just incidental based on the way we shard users, we probably don't have to talk about it here. The important thing is that when a user is queued using this method, they will be checked soon to see if they should receive email.