Yes, email marketing is part of content marketing. Content marketers are using emails to promote relevant content to their subscribers. This can be long-form blog articles, videos, infographics, and more. Those who view the content are likely to remember it better than if they had simply read about it on a blog post or social media page. Ultimately this increases the chances that they will convert into a lead and put their information in your email list.
Email marketing is an effective supplementary strategy because it provides another avenue for recipient promotion. Adopting both marketing tactics will allow you to reach your audience in multiple ways so they get all their questions answered quickly.