Why can we send emails but can't receive any? Keep getting the following message:

"The recipient's e-mail address is invalid. Microsoft Exchange will not try to redeliver this message for you. Please check the recipient's e-mail address and try resending this message, or provide the following diagnostic text to your system administrator."

Tried sending it to our Zoho Mail from various other addresses. Keep getting the same message everywhere. 

We need urgent help please! We need it sorted it out today!

Thank you.