A message on Dave Farber's Interesting People list complained that Comcast was blocking mail forwarded by DynDNS, a popular provider of DNS and related services for small-scale users.
… Wholesale blocking of all mail intended for customers from a particular intermediate distributor, merely because they route it through an external service that adds value.
In response, I opined:
Actually, they're blocking it because a lot of it is spam. This is a problem that every mail forwarder and every mail system encounters; the only unusual thing here is that DynDNS is whining about it. It's yet another way that spammers have broken the mail for the rest of us.
Traditionally, like 10 or 20 years ago, systems that forwarded mail, passed along everything that showed up for the forwarded address. But today, when upwards of 90% of all mail is spam, if you do that, most of what you're forwarding is spam. I can tell you from experience, since I run systems on both ends of forwards (I'm email@example.com among other places) that no matter how clearly you explain to people that the forwarded spam is stuff they've asked for, they will still complain about it, and the automated systems that large ISPs have to use to maintain their spam filters will correctly mark the forwarding IP as a spam source.
The usual next suggestion is that the forwarder should put some sort of flag into the mail to mark it as forwarded so don't blame us. That doesn't work either since any mark your forwarder can make, spammers can also make. Manually maintained lists of known honest forwarders simply aren't practical at the scale of an ISP like Comcast. (For that matter, it's not very practical for my tiny 1000 user mail system, either.) I've adjusted my system so that if a user requests mail be forwarded elsewhere, it runs the mail through SpamAssassin and only forwards stuff that isn't marked as spam. Yeah, you might lose some real mail that way. It's another reason that spam stinks.
The real way out of this is to realize that forwarding is a lot less useful than it used to be. These days, every popular Mail User Agent (MUA), including the big web mail systems, can easily be set up to collect mail from multiple inboxes so rather than doing forwards with SMTP, you collect mail with POP or IMAP, and as a free bonus, it's pre-sorted by inbox.
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