https://techcrunch.com/2019/04/02/inside-a-spam-operation/

For ten days in March, millions were caught in the same massive spam campaign.

Each email looked like it came from someone the recipient knew: the spammer took stolen email addresses and passwords, quietly logged into their email account, scraped their recently sent emails and pushed out personalized emails to the recipient of that sent email with a link to a fake site pushing a weight loss pill or a bitcoin scam.