Retina image
An image saved at twice its display size so it stays sharp on high-density screens.
High-density (“retina”) screens use two or more physical pixels per CSS pixel. An 80-pixel logo saved at exactly 80 pixels looks soft on them.
The fix is to save the image at 160 pixels and set width="80" in the HTML. Email clients then scale it down, and it looks crisp everywhere.
Always include explicit width and height attributes: classic Outlook may otherwise display the image at its full file size.