Yahoo fixed erroneous rejection problem

Yahoo announced over the weekend that they fixed their rejection problem. It may take some time to filter out to all their MTAs, but they do believe the issue is resolved.

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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.
Where does this fit in with what Yahoo is doing? Yahoo is not keeping track of the mail it rejects and is not reliably allowing email through on the second attempt. There are a couple reasons why Yahoo is deferring mail.

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Odd Yahoo Bounces

A number of people are reporting seeing a new bounce from Yahoo. “smtp;553 Mail from x.x.x.x not allowed – [10]”. My clients have been asking and other people have been asking about this. It seems that something is changing at Y! More information as I hear it.

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Yahoo delivery problems

Over the last week or so a number of people have mentioned problems with delivery to Yahoo. It seems that some emails are being erroneously rejected. Earlier this week, Yahoo posted a message to the Yahoo Postmaster announcement list saying they were aware of the problem and were working on fixing it.

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