Why should we not rely on email?

Consider a situation. It is a Friday after noon. You (A) and your friend (B) decide to have a dinner together in the night. Both of you decide that you should dine at Little Sisters (LS). You two decide a meeting time 8:00 pm at LS. A and B like to talk to each other but the problem is that both of them don't like being the one initiating a phone call. So A would prefer writing an email rather than calling if s/he wants to communicate a small piece of information to B. At 6:30 pm, A gets to know that LS is closed that night. Now A writes an email to B that they should dine at Punjabi Dhaba (PD) instead of LS. A reaches PD at 8:00pm and waits for 10 minute but finds that B doesn't show up to PD by 8:10pm. Now A starts thinking. --- Did B ever receive my email. If B did not receive the email then he would go to LS. Should I go to LS and from LS take B with me to PD. What if B actually did receive the email but is late in reaching LS due to some other reasons. In that case I should wait for B at PD----. A also thinks about what B might be thinking, if B has gotten the email. --- B knows that their dining place is changed to PD. But B doesn't know that A knows that B knows that their dining place has been changed ---- .

This is the famous problem of information not being a common knowledge. A and B may both know a fact(F). But does A know that B knows F. If A doesn't know whether B knows F then they don't share the same common knowledge.

This is what happened in the dining plan of A and B. B got the email from A. So both A and B know that their dining place has been changed to PD. But A doesn't know whether B knows that their dining place has been changed to PD. Similarly B doesn't know whether A knows that B knows that their dining place has been changed to PD.

Conclusions: Don't write email to plan dinners :D. Email where the sender is not sure about receiver actually receiving the email should not be written at all. Use your phones people - a bilateral communication device.