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?