A few weeks ago, NetworkWorld posted an interview with Mark Risher of Yahoo. In it, Mark talked about how Yahoo had no plans to outright block or refuse any unauthenticated email. Of course authentication will be a large part of their decision making for incoming emails but they cannot just refuse to accept mail that is unauthenticated, because there are times when unauthenticated email is the most important mail to their recipients.
A lot of marketers often seem to forget that they are competing for time and space with other, non-marketing, types of email. Email from friends and family and discussion lists are both more important to most people that the latest and greatest email advertisement. These are the emails people want to receive, the ones they open, read and respond to.
In terms of authentication, right now the majority of wanted emails are unsigned with DK or DKIM. Sure there are the early adopters who are using DK/DKIM to sign their emails, and a few large ISPs have started signing outgoing email. But until the vast majority of wanted email is actually signed, recipient ISPs are going to have to accept unsigned email.
Looking forward, even if all of the ISPs sign email sent through their SMTP servers, there will still be some fraction of desired email that will be unsigned. Individuals and small businesses who choose to run their own mailservers may not sign email. Even though these servers make up a tiny fraction of total email, they make up a much larger fraction of wanted email. ISPs cannot block this email without angering their customer base.
Marketers should not be concerned about ISPs blocking unauthenticated email, as it is extremely unlikely that any major ISP will do that. Marketers should focus, instead on making their email relevant and wanted by the recipients. I have been recommending clients plan to have all their outgoing emails DK/DKIM signed by the end of 2008.