Do you expect your technology to work reliably 100% of the time? The problem is that technology relies on the interaction between multiple complicated systems and there are a lot of points of possible failure. So, users should expect that things may not work occasionally.
Sending and receiving email is a good example. It works really well 99.9% of the time. However, just like the US Postal Service can occasionally delay or lose a letter, your email can get delayed or lost. So if you send an important email, ask the recipient to confirm receipt or follow up with a phone call.