need help for my Email

there are somethings go wrong.        

       my domain is mpcontractor.com and i setup outlook mail for my domain .

       i don't know what happen to my employee email account *** Email address is removed for privacy *** , this email account can send and receive ok. but some of my customers use this email *** Email address is removed for privacy *** feedback that they send email to *** Email address is removed for privacy *** but she does not receive the mail from them, and the mail does not return to *** Email address is removed for privacy *** if it does not come to destination.

       Please help, because i don't understand what happen to my employee email account.

Thanks for your help.

Minh Phuong.