Email receiving problem

All of the email accounts under the domain facing a very strange issue. When we are sending mail to a person's mail account, he/she gets the message, But when he/she tries to reply on that mail, he/she gets the error : Address not found Your message wasn't delivered to apontexc@example.com because the address couldn't be found, or is unable to receive mail. The response from the remote server was: 550 No Such User Here
Please See the Screenshot from here : [removed - please attach images directly to the thread] Also here is the detailed log from another mail account: Diagnostic-Code: smtp; 550 No Such User Here
How can the issue be resolved? Please guide me. Will appreciate it.