External recipient received a code

Email from an external recipient containing images in the body of the email was replied to from Outlook App on the phone.
The response was received by the external recipient as a code (replacing all of the content).

Started as:

Content-Type: text/plain; charset="utf-8"
> Content-Transfer-Encoding: base64

Some code text in between

Ends with the text below
Content-Type: text/html; charset="utf-8"
> Content-Transfer-Encoding: base64
>

Does this mean that the actual email was of HTML format and the recipient's client viewed it as plain text ?

Need to understand if there is anything we can do to rectify this