My emails are hosting on Office 365. Users communicate with recipients of many different domains hosted on different platforms. When a user of my domain try to send email to external recipients of different domains. Few of the recipients of external domains got the email but some of them doesn't. For example: My domain is abc.com. A sender of abc.com sends emails to recipients of def.com, xyz.com, pqr.com, klm.com. Users of def.com and xyz.com got the email but users of pqr.com and klm.com did not get the email. The sender does not get NDR. I ran message trace in O365, it shows that message has been delivered to all recipients. We have more than 50 clients having their own domains and more than 20 domains are not getting emails from us.