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Over the last week there has been a lot of news around some clients have seen an increase in the 4xx errors coming from Yahoo!. There have been reports that Yahoo! was under some type of attack from various spamming organizations and that they were using this method to fight off those attacks. Others believed that Yahoo! had officially put in a grey-listing methodology as part of their anti-spam efforts. Neither of these beliefs are correct.

Yahoo! has verified that they have recently put in new, more aggressive measures to fight illegitimate mail coming into their servers. While they have acknowledged that this may be have some affect legitimate mailers they are working on fine-tuning these systems. Their goal as always is to make sure that the legitimate mail that people want to receive gets to their end users, and that which they don’t is blocked. According to Yahoo!, any legitimate mail that has been received but not yet have been delivered will eventually reach the intended recipient as their inbound mail queues drain. They have asked for everyone’s understanding during this time and hope to have the issue resolved soon.
If you're seeing 451 4.70 [TS01] or [TS02] errors you should wait at least 4 hours before trying to redeliver the mail.

As always if you have any questions please feel free to reach out to our support group or for this particular issue you can go to http://postmaster.yahoo.com.