Excerpt from this article:
When researchers compiled a huge database of the digital habits of teams at Microsoft, they found that the clearest warning sign of an ineffective manager was being slow to answer emails. Responding in a timely manner shows that you are conscientious — organized, dependable, and hardworking. And that matters. In a comprehensive analysis of people in hundreds of occupations, conscientiousness was the single best personality predictor of job performance. (It turns out that people who are rude online tend to be rude offline, too.)
I’m not saying you have to answer every email. Your brain is not just sitting there waiting to be picked. If senders aren’t considerate enough to do their homework and ask a question you’re qualified to answer, you don’t owe them anything back.