I recently went to the mind/spirit exhibition and watch people queue to see psychics.
I suspect that many visit a psychic to get more certainty. It is very important to some people that they can be certain about some things.
If you ever get a hint that certainty is important to someone, make sure you respect this need. Reassure them that you will deliver what you promised. If you tell them you will call later in the day, make sure you do.
Imagine the effect of telling someone who liked certainty that you would personally deliver an important document or order on the same day… and you did. Some people like guarantees, assurances, promises. They seek certainty in a world where little is certain. It is probably a remnant of our very old brain.
So give people as much certainty as you control. Call them back. Send the email today. Give them what they need in an uncertain world.