Well, recent changes just changed email to <source>@packages.debian.org, so I'm not convinced this is a bug in dak itself.
Could it be the small delay while processing mails to <source>@packages.qa.debian.org (which are then forwarded to <source>@packages.debian.org) could have hidden this for ages? Anyway, does the solution to use dispatch@p.d.o works for source NEW, or for binary-NEW too? -- Cheers, Luca