There are several plausible reasons why you cannot receive email from a particular sender. It is probably best to verify a few things first:

Does the sender have the right email address?

The best way to check if the sender has your correct address is for you to send them an email and ask them to click on the "reply" button and send a reply back to you.

You have not received the reply

If you sent an email and the sender replied and you still have not received the reply, chances are the email is being held in the "greylist". This will occur if this is the first time the sender is sending to you. There is a wait time for email to pass through the greylist filter. We suggest you wait about an hour for the system to process the email.

Still no reply - it must be the sender's server!

If you still have no reply, then it is very likely that the sender has a mis-configured email server. This is a somewhat common occurance, especially with Microsoft Exchange Servers. The sender will need to notify their IT people.