[mnw21cam]

I would point out that there is nothing in the SMTP standard stopping anyone from sending stuff from any email address.

[glangdale]

While a good deal of our entertainment circa 1993 (Computer Science Honours, Sydney University) centered around exploiting this delightful fact (close second: unattended terminals), I had long since figured that forging emails off a open SMTP port wasn't really still A Thing.

[mnw21cam]

Oh, it most definitely still is A Thing. There have been a few notable attempts to stop it happening, but none of them have really worked completely, and they only apply to some from-domains.

[sam_lowry_]

O'rly?

Try sending a mail from [e-mail] to anyone you know.

[TallGuyShort]

Microsoft operates some of the more well-known of those from domains.

[OJFord]

Use your own domain for email and watch the 'hacker' spam roll in.

[RandomBacon]

I do, and get no such spam.