I came across an issue where a client received 3 reminder emails within 24 
hours and is incredibly upset because she feels like she is being rushed.
I attached a screenshot below.
This isn't a normal occurrence is it?
I was curious normally how much time is between each reminder email sent? 
Like when do we send each of these out?

Looks like this is because of the postal service data. If you look here it shows that her fix arrived at the military post office on May 9 and they didn't deliver it to her until May 27: https://tools.usps.com/go/TrackConfirmAction_input?qtc_tLabels1=9405510899535005284109

Our systems don't handle that unusual scenario because with normal addresses we have a 3 day delivery guarantee. Apparently they either have a customs delay or she wasn't available to receive the package until several weeks after it was delivered to the military PO. Here's the breakdown of the details:

The first reminder she has there should be sent 10 days from ship date, which should have been May 8
The first overdue (truant) email should have been sent May 12 (14 days from ship date)
The final overdue (truant) email was due on May 19 (21 days from ship date.) This one is only sent if the first one is sent, but they ended up happening the same day because they were both due and the first one happened a minute prior to this one.

All of these emails are only triggered if the shipment is in the Delivered state. If for some reason her shipment was in Outbound, Missent, or something like that for a while and just got picked up by USPS tracking then it would mean they could all be triggered at the same time because the logic for all of them is roughly "find everything in Delivered that was shipped X days ago and alert them." They are actually sent by different apps even (the first by hellblazer, the final two truant ones by kingmob.)

I'm not sure exactly the right solution to fix this in the future. One bit of good news is that we are centralizing all of our email sending to be one application so it could be easier to handle a scenario like this. We also need to be smarter at looking at the whole of reminder emails when determining if it's time to send the next one rather than running on the assumption of everything happily following the days we expect things to happen. So I think we need to do some brainstorming about these on the eng team. I'll CC the kufak team so that we can have a more well rounded higher level discussion.

For now, maybe an explanation will help her feel a bit better?