A user have iPhone x, When he send email from Iphone native app he get the bounce back message. and email working fine on OWA and outlook.
did you tried reconfiguring the user account In the device?
Is this happening for an email he sends or he gets the bunce back for any email he sends to anyone from his iPhone?
try to check the message trace from EAC for the detailed information regarding that email.
You may also share the bounce back for further analysis which he is getting.