Why is my client not receiving invoices

If you have sent and it says delivered within the finance portal even though the client is still not receiving it

That can usually mean two things:

Their inbox is full and they have no space to receive emails - the solution is to empty and clean up the inbox

or

They have blocked your emails - The Solution - The client has to unblock the IP - https://docs.microsoft.com/en-us/office365/securitycompliance/removing-user-from-restricted-users-portal-after-spam