Change the format of a reply or forwarded message to fix errors when sending

You can change a plain text message to HTML.

In the message, click Reply, Reply All, or Forward.
If you’re working in the Reading Pane, click Pop Out.
In the message window, click Format Text > HTML.

Other options

If you want to change the message to Rich Text Format, click Rich Text.

Outlook will now allow you to send the email

https://support.microsoft.com/en-us/office/change-the-message-format-to-html-rich-text-format-or-plain-text-338a389d-11da-47fe-b693-cf41f792fefa