Handoff could not be completed." The handoff feature works fine when simply viewing emails. It also works fine with Safari. I'm pretty sure that the problem is with the iPhone, as the handoff feature works just fine between my Mac and my iPad, but not between the iPhone and the iPad. Basically, it fails only with the iPhone. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.