Using New Outlook on Windows for personal email, calendar, and contact management
I solved it by writing my email in Word 365 and adding a horizontal line from the Borders dropdown menu. Then I Copied everything and pasted it into the email in new Outlook, and it worked.
I am continuing to use New Outlook as last year I found the Classic version suddenly stopped recording my Sent emails, and that was a disaster.
Neville Fursdon