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.

• Shedding load in a generic and non-specific way.
• Shedding load by temporarily refusing mail from specific IPs.

In the first case, the shedding of load means nothing more than Yahoo is shedding load. There is not really anything the sender can do to compensate for this, nor is there any thing the sender is doing (except possibly send mail to Yahoo at the same time as the rest of the world) to precipitate the blocking.
In the second case, these are more specific refusals and there are things senders can do to minimize the deferrals.

1. Have good address collection practices.
2. Have good data hygiene (prune your bounces)
3. Do not send more than 5 emails per any single connection
4. Do not open too many connections from any single IP address
5. Honor unsubscribes promptly
6. Apply for whitelisting

Even the best mailers sometimes see deferrals at Yahoo. However, because Yahoo is using a temporary rejection, unless there are significant problems with your mailings, the mail will get through.