Check the sender's and recipient email format on outlook end.
If the user sends in plain text it will be delivered in plain text.
You have mentioned recipient is plane text even in webmail portal. change the settings on the recipient side also.
Hi,
For your question, I want to confirm whether only one special domain experience this issue, is it internal mail flow?
For Exchange server, we can configure the content conversion for remote domain, user and Outlook. Also, check the transport rule. For your reference: https://technet.microsoft.com/en-us/library/bb232174(v=exchg.150).aspx
If this issue persist, please enable content conversion tracing in the Transport service on the Mailbox server, or in the Mailbox Transport service on the Mailbox server. Please refer to: https://technet.microsoft.com/en-us/library/bb397226(v=exchg.150).aspx