Can't send email within same domain

Can't send email within same domain

Hi, I have configured MX records and seem to to be able to send and receive email fine. But I can't send email to other email within the same email domain. Eg, I can't send email from a@domain.com to b@domain.com.  Keep getting Undelivered Mail Returned to Sender error returned email. The content as followed

This message was created automatically by mail delivery software. 
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. 

[Email] INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.1.1 The email account that you tried to reach does not exist. Please try 
            5.1.1 double-checking the recipient's email address for typos or 
            5.1.1 unnecessary spaces. Learn more at 
            5.1.1  https://support.google.com/mail/?p=NoSuchUser  a34-v6si22853518pld.249 - gsmtp 



Received:from mail.zoho.com by mx.zohomail.com 
    with SMTP id 154112119911015.928991908287344; Thu, 1 Nov 2018 18:13:19 -0700 (PDT)