Windows 10 mail client not sending email trail

[bantonia]

Have an issue where the Windows 10 mail client only sends the currently edited email text and not the whole email thread. This doesn't happen with Outlook, of various varieties, and only one user is affected but that user is reluctant to use any other mail client.

Any clues how to fix this without using another mail client? It was working but stopped fairly recently. Company Portal was installed to allow the Windows 10 mail client to work with 365.