"Email marketing software that drives sales. Create, send, and track email campaigns that help you build a strong customer base."
They don't have 40 million users. They have 40 million targets.
Of course they don't get many complaints. If you search for "zoho opt out", you get sent to a page with a HTTP 400 error.[2]
[1] https://www.zoho.com/campaigns [2] https://help.zoho.com/portal/kb/articles/what-does-email-opt...