You may have been unfortunate enough to be in the situation where you've missed deadlines or received communications from clients after the information was useful to you. The temptation is to rely on email for all your communication needs, after all, 99.5% of the time it does the job doesn't it?
Email Was Never Meant To Be Instant
We've all come to expect that emails will arrive in seconds, if not a few minutes, but if it's over an hour then we assume something has gone wrong,
There is a lot of software and hardware in between that your email has to pass through to get from the point of origin to its destination. This can be server hardware, software, routers, switches, cables, power systems, or even your own computer, there are many many potential points of failure along the way. If any one of these has an outage or is overloaded, a delay can easily occur.
Most of the time it's out of your control
There is usually little you can do to rectify any of these errors, as most are on the other side of what is within your control, and email has been built to allow for such delays, with most servers trying to resend an email for at least 24 hours before it will give up, some even up to 7 days! Using an instant messaging system such as Skype for Business is much more efficient for real-time communications.
In the end, even with the occasional delays, email still beats writing letters on paper, stuffing them in envelopes, licking stamps, and waiting for the postman to get the letter where it's going, but if it's urgent either Pick up the phone, or if it's a critical business need then you need to get a system that is suitable for Real Time Communication, or you will come fall foul of email delays.