Have a weird issue where we are not receiving emails from a foreign client (client is based in South Korea, we are US based) to our main DOMIAN.COM email accounts which are an Office 365 setup. We, however, are receiving emails from this client to our backup, DOMAIN.NET, account which is Google setup. Our client will send an email to the .COM address and copy the .NET address. We will receive the email in the .NET account. Checking the spam filters and log files there is no indication that the email ever reaches the O365 server meaning we would never know it was sent without the .NET backup.
I have checked the MX records on my name server, moved to a new name server, tested my domain and cannot find a reason as to why these inbound emails will not reach the server.