These days most people rely on email to carry out their business and communicate more readily but you shouldn't assume that emails will arrive in the other person's Inbox seconds or minutes after they've been sent.
Sometimes there can be a good hour or two delay and the person may not be around to respond at the speed or in the time that you'd like. It depends on the ISP, route taken, time of day, email settings, temperature of the ether, climatic conditions :) ... etc. Servers crash, connections go down - while the majority of emails will get through safely, emails can go astray and never reach their destination. They may get filtered out by over enthusiastic spam filters, blocked by an ISP, mislaid or just lost in the volume of emails that people receive (on average 100 a day).
You can use the 'request read receipt' option when you send an email but again that doesn't always guarantee delivery - some people have this switched off or if it's set to autorespond - they may have received it in their Inbox but that doesn't necessarily mean that they've read it.
If it's really urgent, pick up the phone. Don't rely solely on email to deliver your message.