Back in the early 2000's I worked for a company that was the lead firm doing analysis of email spammers for later targeting by the law firms of the big free email providers.

The phrase "be liberal in what you accept, and conservative in what you send" certainly applied to SMTP which led to all kinds of craziness in spam.

For example:

- Emails with multiple "From" fields in the headers

- Totally bogus relay chain data

- all kinds of other garbage in random fields

- sender IPs that were faked etc

This was also around the time that spammers were switching to using bas64 MIME encoded images to get around text filters.

The company I worked for actually got pretty good at analysing the spam thanks to a combination of a IMDB style web based browser for the spam I put together and good old fashioned "leg work" to track the owner of URLs, merchant accounts etc. That in turn led to some of the spammers actually shutting down.

More details here: https://twitter.com/alexpotato/status/1208948480867127296