As long as it doesn't cause providers to *negatively* weight mails sent from other servers, I like the idea.

However, if it would cause providers to negatively weight emails that weren't sent from DW's servers, I'd say no, because lots of people might send email from their @dreamwidth.org address via their own mail servers if they don't want to give out their real email address, and it would be bad for them to be marked as spam simply because it didn't come from Dreamwidth's own mail servers.