Many email servers report the IP of the incoming SMTP connection when you use a client (e.g. mutt, Thunderbird, Apple Mail, or Outlook) as an "X-Client-IP" or similar raw header; often also the envelope "from", which may be different than the message "from" which is the only one usually shown.

It's not that IPs are very private - sending an email to someone often tells a lot about you. But having all that info publicly scrapable seems unexpected to me.