Substack is raising $65 million in new venture capital funding that would value the company at around $650 million, Axios has learned. Existing investor Andreessen Horowitz is leading the round.

Why it matters: Substack, which provides a platform on which writers can publish paid email newsletters and keep most of the revenue, has seen its popularity soar.

• Many of Substack's best-known writers are professional journalists who sought to become their own bosses, while others are simply seeking an outlet for their expertise.

Complications: Some of Substack's practices have become controversial, including who it chooses to pay directly and its relatively loose content moderation policies.

• It also could have tough competition from Big Tech companies like Twitter and Facebook, plus rival online publishers like Medium.

Andreessen Horowitz declined comment, while Substack hasn't yet responded to Axios' inquiries.

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