Marissa Mayer. Ethan Miller/Getty Images Marissa Mayer just gave birth to twins, but she's not getting a pass from Eric Jackson, a fund manager who just launched a big attack on her as Yahoo CEO.

Jackson sent a 99-page presentation to Yahoo's board outlining his case for why Yahoo should drop Mayer as CEO and find new management.

His presentation is also a warning shot that Yahoo should not sell its core business right now.

In Jackson's view, Yahoo's core business is undervalued after being poorly managed by Mayer. Selling today would mean selling it at a low point. He thinks selling Yahoo right now would only enrich some private-equity people while current shareholders would miss out on upside.

As a result, Jackson isn't just taking on Mayer — he's also taking on the hedge fund Starboard, an activist investor that is attacking Yahoo.

Jackson's fund, SpringOwl, isn't a major shareholder in Yahoo. SpringOwl is only a $300 million fund. But his plan is to round up major shareholders to rally around his plan. He says several are already supportive.

We have the full presentation below.