[bitfolk] System aliases and DMARC

For many years I've run a poor-man's mailing list through /etc/aliases on my VPS. Before you start breaking out the flaming torches and pitchforks, it's very limited in scope; it forwards only within my immediate household, albeit to mailboxes hosted by gmail and hotmail. I've just learned that some mails to this alias are being quarantined or bounced at their ultimate destinations. They're passing SPF (because envelope-from is postmaster@ my vps) but failing DMARC (the external From address isn't being rewritten). When the sender has full DMARC enabled, we lose. Drat. My VPS is running Debian with exim4. I think I might like to rewrite "From: foo(a)bar.baz" to something like "From: postmaster+foo_bar.baz(a)my.domain" in order to satisfy DMARC, but only when forwarding via this particular alias. I'm not readily figuring out how to do this, and am leery to tangle with Exim's rewrite rules anyway. Would anybody care to venture whether this is possible? a good/bad idea? alternative solutions? I am looking for a least hassle, least maintenance answer, ideally at little or no additional cost (hence /etc/aliases has served well for a long time). On a unicorn, naturally :-) (No I don't run mailman - I used to but I found it rather tiresome to set up, feed and water.) Thanks Ross