But if someone sent mail purporting to be from @dreamwidth.org, and from a home IP address, my organisation would reject it anyway. We don't accept mail from home networks, and while I enforce a stringent ruleset in that way, it is not rare.

I hope you mean you would block mail where the first mailserver was on a home IP address. Plenty of people send mail from a home IP address, but it's incredibly rare that any good mail would be sent where the mailserver itself was on a home IP address - instead, those mails would be mostly sent through their ISP's mailserver.

I agree with blocking in the mailserver/home IP case, but not blocking everyone who just happens to send their mail from home. How on earth would you check for that, anyway?