Emails sent via Office.js add-in not rendering correctly in inbox

Hi everyone,

I’ve developed a custom Outlook add-in using OfficeJS and I’m encountering an issue related to rendering HTML content inserted into the message body via the setSignatureAsync method.

The problem specifically occurs **when a message is composed in the New Outlook experience and then received in Old Outlook. In this case, the HTML content does not render correctly in the recipient's view.

Here are the scenarios I've tested:

Old Outlook → New Outlook: The email renders correctly.

New Outlook → New Outlook: The email renders correctly.

Old Outlook → Old Outlook: The email renders correctly.

New Outlook → Old Outlook: The email renders incorrectly (HTML formatting issues).

Has anyone encountered a similar issue or found a workaround for consistent rendering between Outlook versions? Any insights or guidance would be greatly appreciated.

Thank you!