Short answer
HubSpot's KB: for custom coded emails, adjust image spacing with hspace/vspace or tables. Set font family, size, color and line height, since Outlook adds its own styling. Some Outlook versions show a horizontal line between sections; the KB lists steps.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Test in the Outlook versions your audience uses.
3. Related
Email client previews and dark mode.