You're wrong. Most SMTP requires IP in the header, gmail does not. This causes contempt between certain groups, as while it's a boon for personal privacy, it often hinders investigations.
Here's a video with Mike Hearn giving a talk about spam. At the end and middle are mentions about gmail's lack of IP from sent emails, the end specifically from a network admin expressing his grievances about Google's efforts being a hindrance to law enforcement.
Interesting. The link I posted above must not be the one I read a few days ago on HN (or somewhere else?), cause it was actually posted several years ago on HN. Let me find what I read recently...
Here's a video with Mike Hearn giving a talk about spam. At the end and middle are mentions about gmail's lack of IP from sent emails, the end specifically from a network admin expressing his grievances about Google's efforts being a hindrance to law enforcement.
https://ripe64.ripe.net/archives/video/25/
It is , however, standard practice for most SMTP providers to do this.