office 365 can't receive email from a specific user

There is a network with firewall and office 365. There is a user let say user_A who can't receive any email from a specific user let say user_B. When i say can't received any email i don't mean that the email is going to junk mail but i mean that the email is not received neither in inbox and or junk box.

Other users are able to receive this email

How to troubleshoot this issue? what step are needed to do?
Who is Participating?
Jackie ManConnect With a Mentor Commented:

Are you o365 admin?

If yes, you need to do a message trace first.
Jackie ManCommented:
Have you just done a migration to Office 365 Exchange online from on premises Exchange server?

Both User A and User B are old users before migration? User B is not on the same domain of User A?
Jackie ManConnect With a Mentor Commented:
If my guesses above are correct, when User B sends an email to User A, the email address of User A is resolved to legacy DN email address of the old Exchange server and the email cannot be delivered.
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GasAuthor Commented:
No (maybe was migrating few years ago) not now.
GasAuthor Commented:
I will do a nessage trace. What next?
Jackie ManConnect With a Mentor Commented:
What next?

You will know whether the missing email ever reached the mail server of your Office 365 or nor.

If there is no such email, it is the sender problem and they need to do a message trace at their side and they need to talk to Microsoft to troubleshoot the problem if their mail server did try to send the email but failed.

If there is such email at mail server of Office 365 but User A did not receive it, you need to check whether there is a server-side rule or client-side rule to move/delete the email.

If there are no such rules, you need to check your firewall log or web proxy/filter log or client-side antivirus / Internet security log for blocking the email.
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