A few times recently I've been contacted by email to cancel a meeting at short notice or request an urgent piece of information needed within the next couple of hours.
Now, if I'm following my own suggestions, I won't be checking my email every few minutes and I may even be out of the office and not get to it until later. It may even be switched off for a few hours if I'm with clients or working on something.
What is it about email that it makes people forget there are other ways to communicate and use it either as the easy option or as a means of 'instant communication'? It's not! Email is far from reliable and you can't and shouldn't assume that just because you've hit send, it's immediately appeared in their inbox and they'll be reading it and responding in the next few minutes.
If it's urgent, important or time critical - pick up the phone and remember that they're timeframe and level of urgency may not be the same as yours.