Cannot Send or Receive Emails

Cannot Send or Receive Emails

Hi,

I have 2 email addresses (users) set up under the domain: zen-black.com

I have sent and received emails from both of them to outside (other) email addresses and it works fine.

But if I send emails between the two of them (users) it doesn't work.
It says sent successfully in one emails sent folder, but nothing appears in the inbox of the other...
(its the same for both of them)

Its bizarre!

Help!!