[Mailman-Users] (relatively) new DMARC issues - and Gmail

Richard Damon Richard at Damon-Family.org
Sat Mar 31 14:50:56 EDT 2018

On 3/31/18 2:31 PM, Lindsay Haisley wrote:
> I've been working with a From-rewriting tool with code taken from
> Mailman (thanks, Mark!) and discovered a couple of things which I
> thought deserved posting about. I expect that they peripherally affect
> Mailman, too.
> At some point Amazon (amazon.com) started publishing a DMARC
> "p=quarantine" policy, which means that any email which gets redirected
> and hits my dmarc_shield piece is going to have its From address re-
> written to "postmaster at fmp.com" (fmp.com has a proper SPF record).
> I don't know what Gmail's policy is with regard to "p=quarantine" -
> whether it rejects such email outright or relegates it to the
> recipient's spam folder. I know that if the sending site publishes
> "p=reject", redirected email is refused by Gmail at the front door.
> I'll have to test the "p=quarantine" behavior.
> Here's the really annoying thing. My dmarc_shield processor rewrites
> the From header as per SOP for Mailman with the proper switch turned
> on. The From header address becomes "postmaster at fmp.com" with the
> original From address in the address comment (from xxx at yyz.com). If
> the email didn't already have a Reply-To address, the original From
> address is inserted as the Reply-To address. If a Gmail user replies to
> such an email, the reply goes to the Reply-To address, but Gmail
> **whitelists** the From address! Thereafter, any email which comes in
> with a munged From address is accepted, bypassing Gmail's otherwise
> pretty good spam filtering. I'm noticing a lot of spam email going out
> with From addresses for which a DMARC "p=reject" policy is published,
> which means that any such spam redirected to the Gmail user via FMP is
> also whitelisted. Bah! It's a fucking war zone out there!
> The only possible solution here would be to randomize the username
> portion of the rewritten From address, which makes the email look more
> like spam, and the Gmail user would end up with a whole lot of useless
> whitelisted address which would need to be deleted. Not to mention the
> fact that FMP's mail server might be blocked from sending ANY email to
> Gmail.
To me the issue sounds like why is fmp.com forwarding spam?

If this is a case of fmp.com offering forwarding mailboxes to users, who 
might be using gmail as a final destination, then yes, fmp needs to try 
to be as good at detecting spam as gmail or users need to accept the 
increased spam levels.

Another option is to deterministically munge the from address so every 
incoming email address gets a unique fmp address that it represents (it 
doesn't have to be absolutely unique, mostly unique is likely good 
enough), something like replace the at with _at_ and add a tail wart 
like _dmarc at fmp.com (so you can have other addresses an not worry about 
possible overlaps with those) and use that as the from address. Then a 
reply will only whitelist that specific original from address.

Richard Damon

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