If everything else is fine, and both users can not only send e-mails to an outside domain, but receive from an outside domain also (you didn't tell about that), it should be something inside the mail server. So better check that.
email not delivered to same domain
i have a client who hosting their email and are unable to receive email from same domain. email@example.com are not receiving email from firstname.lastname@example.org 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?