I don't receive everyone's email or they don't receive mine.

Its important to understand how email works, and its very similar to regular US Postal Mail.

When an email is sent, it bounces through anywhere from a few to possibly over a dozen different stops, just like postal mail may go through different sorting facilities and trucks.
Eventually, if everything goes smoothly, it arrives in your Electronic Mail Box which is just like the Postal Mail Box sitting outside your house.
You need to remember that just like the postal mail box outside your house, if the mail gets stopped anywhere alone the way for any reason, your mail carrier wont have it and wont put it in your mail box.  If that happens and it was stopped somewhere along the way, there is nothing you can do as all you have is your mail box sitting there waiting to receive anything that gets put in it.

The question is why would it get stopped along the way?

A few possibilities are that your spam blocker may have thought it was spam and deleted it or put it into a special folder for spam.  This could be something done on your computer, or even something done by your email provider.
Another possibility is that the sender (you if someone is not receiving your email or someone else if your not receiving their email) may have been black listed.  For some reason they are being blocked because they have been reported to one or multiple spam servers as a spammer. You can read more about there by clicking here.
Then again it could just be a technical problem and was lost along the multiple stops it made on its way to its destination.
Like with the US postal service who handles millions of letters a day, email is 99.999% reliable. Usually the problem is legitimate. Meaning, one of the first two possibilities was the cause. Its very rare that it just disappears due to a technical reason.

What to do about it:

1) Did the sending get a message back? An undeliverable error? If so, the cause is almost certainly listed it that response in technical terms. Try reading through it or send it to your email provider to have them examine it.

2) Have them send an email from a different account (Yahoo, Google Mail, AOL, Etc.) and if that comes through then their other email address is probably being blocked because they got on some spam list. Read more about that here.

Almost always we find that one of those two are the cause.