email not delivered to same domain

i have a client who hosting their email and are unable to receive email from same domain. user1@domainx.com are not receiving email from user2@domainx.com or vise versa . but they can send email fine to outside domain. the email are sent successfully and no error are returned. just like its getting lost somewhere. what can be causing this? does anyone have personal experience with them?I would like to hear other thoughts on this.