Inquiring to see if there's an updated solution to issue. Article was published January 7, 2022.

Outlook adds unintentional line spacing in messages, specifically between lines that should have no spacing and between paragraphs that should have only single spacing. This happens when you receive a reply to one of your emails from someone who uses a web-based program such as Gmail. The issue occurs exclusively in Outlook's HTML mode because of the way the program treats line and paragraph breaks for web-based email clients.


How to fix unintended line spacing in your Microsoft Outlook emails | TechRepublic.