I do business with a firm in Chicago and Milwaukee. One of their computers in Chicago and one of their computers in Milwaukee cannot send messages to me. They both receive my messages but need to use another computer in their office to answer me.
You need to add much more information. (also, I'm pretty sure that this is in the wrong section).
What OS are they running? What Email software? What error messages do they get on the particular machines that are failing to send the Email to you? What is different (OS, email software, sober user) on the machines that can send Email to you? Do they Reply or send new messages to your Email address? Can the machines that can't send Email to you send Email to anyone else? What is the flying speed of an unladen swallow?
But seriously, except for the Monty Python reference you will need to supply at least the information I mentioned before someone can make a reasonable assessment.