Recipients are not getting emails

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.
Shamsher SinghAsked:
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Vasil Michev (MVP)Commented:
If message trace shows the message has left O365, it's really up to the other party to troubleshot this. The message might have been marked as spam or similar, redirected someplace, even deleted. Ask them to run a trace on their end if possible, otherwise we can only guess.

To minimize the risk of false-positive spam/phish incidents, make sure your SPF/DKIM/DMARC records are properly configured.
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timgreen7077Exchange EngineerCommented:
If message trace shows that the email left the O365 environment, then the recipient will need to look on their side also. It could be spam filtering or anything on the recipient side that caught the mail. If you can show that it left O365, the you are clear and there is not much else you can check on your end once it leaves O365. The recipient will need to look on their end.
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Shamsher SinghAuthor Commented:
Thanks Tim
It would be clear to me if there is only one domain not receiving my emails but there are more than 20 clients with different domains started complaining a sudden. Everybody was getting emails a day before.
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timgreen7077Exchange EngineerCommented:
I fully understand but once the mail leaves office 365 you can't control anything else while it's in  route or at the end user destination.  If you confirmed that it left your environment that's the only things you can do at that point.

 You can try to telnet to the problem domains and see if there are any issues.
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timgreen7077Exchange EngineerCommented:
Answered question, closing ticket unless author requests to reopen for further assistance.
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