Also, email delivery in general can be delayed for hours. It’s not common, but there are no delivery guarantees around email. You could reply immediately, and it could arrive in their inbox much later.
This doesn’t happen enough to me that I consider it.
I once built an IdP that vended access one time codes via email. We had to switch to other means because there were a small (but significantly high) fraction of customers who didn’t receive their OTPs before they expired (TTL was 15 minutes). Also even if they came in before the TTL, a user experience that involved waiting that long to get logged in was not tenable.