First of all, the must-haves of any email design system are the brand elements:
- Logo(s)
- Brand colors
- Different fonts and font sizes (plus use cases)
- Icons and trust symbols
- Assets (imagery and key photos from the brand book)
Once you import everything into Adobe XD (or Figma) make sure to add it as separate components so that you can start building your library of editable and reusable elements.