Yahoo delays, part 2

A number of people have posted to various mailing lists and made blog posts pointing to the Yahoo Mail blog post discussing recent problems Yahoo was having with mail. The general feeling seemed to be “AHA! That’s what is wrong!”
Unlike many of my peers, I do not think this explains the delivery problems senders have been seeing while attempting to deliver mail to Yahoo. The Yahoo mail blog article is talking about the Yahoo outgoing mailservers (smarthosts) for their non-webmail users. It is extremely unlikely that these are the same servers used for incoming email.
While I sympathize with everyone who had the AHA! moment and thought their delivery problems were being acknowledged and addressed by Yahoo! I do not think this is really what that blog post is saying.
I am hearing from people that Yahoo is aware of a problem with delayed incoming email, and they are working on fixing it. This does seem to be a broader problem than just bulk mailers, I am hearing from small and mid-size ISPs that they are having significant problems delivering email to Yahoo, too.
For more information about what Yahoo is doing to filter mail check out my previous post Greylisting: that which Yahoo! does not do.

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Greylisting: that which Yahoo does not do

Over the last couple days multiple people have asserted to me that Yahoo is greylisting mail. The fact that Yahoo itself asserts it is not using greylisting as a technique to control mail seems to have no effect on the number of people who believe that Yahoo is greylisting.
Deeply held beliefs by many senders aside, Yahoo is not greylisting. Yahoo is using temporary failures (4xx) as a way to defer and control mail coming into their servers and their users.
I think much of the problem is that the definition of greylisting is not well understood by the people using the term. Greylisting generally refers to a process of refusing email with a 4xx response the first time delivery is attempted and accepting the email at the second delivery attempt. There are a number of ways to greylist, per message, per IP or per from address. The defining feature of greylisting is that the receiving MTA keeps track of the messages (IP or addresss) that it has rejected and allows the mail through the second time the mail is sent.
This technique for handling email is a direct response to some spamming software, particularly software that uses infected Windows machines to send email. The spam software will drop any email in response to a 4xx or 5xx response. Well designed software will retry any email receiving a 4xx response. By rejecting anything on the first attempt with a 4xx, the receiving ISPs can trivially block mail from spambots.
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