[athenot]

The From header has always been spoofable. It's just most ISPs (and Google) chose to disallow it to address low-hanging fruit in the fight against spam.

But anyone can set up their own postfix/qmail/sendmail server and put anything they want as the From.

Or am I misunderstanding the issue here?

[m4lvin]

The difference is that using another fastmail account to spoof [e-mail] will make the email look much more legitimate (DKIM and SPF wise) to other servers than when it comes from your random.emailserver.domain.foo.