With dotDelivery, you can send out your emails to your clients based on the time that they have proven to be opening emails. Lots of high-tech algorithmic stuff going on, but basically, every time they open an email the dotDelivery system tracks that time and builds a ‘normal’ time for that client. Then, as you send out your emails, ti will delivery it to them based on that custom time.

Again, this is a ‘next level’ tool that not everyone will need, want, or have time to implement, but if you are really hard-core about your deliverability, go check it out.