At Rockerbox we're big believers in the Subway Principle.
We defined the principle in our company wiki as:
Subway Principle: If we get an email, but don't want to / can't respond right away with an answer, we respond right away acknowledging receipt of the email and indicating that a response will come. Better for them to know we saw the email than to hang in thin air.
This mostly comes into play when we get emails from clients, prospects or job applicants that we've interviewed (not for cold emails from vendors). The idea being that there's nothing worse for the sender than not knowing if an email was actually received and not having any expectation of when an answer will arrive.
Why is this called the Subway Principle? It stems from an article I read a while back that I'm unable to find (If you know the article I'm referencing please send it my way because I take zero credit for inventing this principle).
The premise of the article was about how you feel when you're waiting on the platform for your subway. If you're waiting with no indication of when the next subway will arrive, you'll get frustrated. You'll get frustrated even though the next subway may be arriving in 2 minutes. Simply not knowing is what causes the frustration.
On the other hand, you're more calm if you're waiting at a platform that has screens displaying expected arrival times of the next subway. Sure you'll be upset if you arrive to a 30 minute wait. But once you get over the initial anger, rather than just sitting around and twiddling your thumbs, you're in a position to figure out how to spend the 30 minutes.
With subways, it's better to know so you can plan than to be unsure. The same is true with customer emails.
Also published on Medium