Yahoo

Yahoo has a complicated filtering setup that uses a variety of techniques to protect their users from unwanted email. They use a combination of rate limiting and temp failing along with bulk foldering as defenses. Yahoo also uses some lists provided by Spamhaus including the PBL, SBL and XBL.
Yahoo provides whitelisting and FBL services. For senders and ESPs to qualify for the Yahoo FBL they must sign their mail with Domain Keys or DKIM. ISPs are eligible for an IP based FBL. Whitelisting is available to senders who fill out an application and qualify.

Delivering to Yahoo

Yahoo limits the amount of mail that can be sent by any sender to 20 emails per connection . Yahoo also uses reputation based rate limiting. Reputation is a measure of how wanted email is. ISPs use a lot of different inputs to measure reputation including: invalid address rates, spamtrap hits, complaint rates, and this is not spam clicks. Each ISP has different weighting they give the different measurements, and their measurements are evolving and changing to keep up with spam. More details available at Sender Reputation.
To reserve capacity for wanted email Yahoo uses a number of different techniques.

• Reputation based ratelimiting. Senders with poor reputation, or with links to URLs and websites with poor reputation, may see their mail slowed down with 4xx rejection messages.
• Limiting the number of emails per connection to 20. After 20 emails have been sent in a single connection, no further messages will be accepted for delivery as their server automatically terminates the connection (without giving an error code).
• Limiting the number of connections per IP. This limit is not published, but empirical observation suggests up to 25 connections per connecting IP is acceptable.

In addition to rate limiting senders, Yahoo uses a number of techniques to prevent unwanted mail from getting to their users. Starting in late 2010, more emphasis seems to have been put on content reputation over IP based reputation. Previously, good IP reputation could override poor content, but this seems to no longer be the case. Evidence also suggests Yahoo is following links and looking at landing pages as part of URL reputation.

Important Links and References

Postmaster Pages

Yahoo has an extensive set of Postmaster Pages with support for senders and ISPs. These include information on error codes, rate limits and best practices. They also have 2 mailing lists, one for announcements from the postmaster team and another for announcements about Yahoo groups.

Feedback Loop

To qualifty for the Yahoo FBL senders and ESPs must sign mail with Domain Keys or DKIM. There is an IP based FBL available for ISPs and hosting companies. Conditions and enrollment information is available on the Complaint Feedback Loop information page.
Yahoo FBL emails are sent in ARF format and Yahoo does not redact any information from the original email. All Yahoo FBL mail is DomainKeys signed.

Whitelist

To sign up for the Yahoo! Whitelist everyone must fill out the Yahoo! Mail Bulk Sender Form. Once the form is submitted, Yahoo will send a confirmation email to the requestor. Yahoo! will then monitor mail for a period of 2 – 4 weeks. During this time mail may be delivered to the inbox or the bulk folder. Based on recipient reaction and mail statistics Yahoo will make the decision to offer whitelisting or not.

Authentication

Yahoo does reject mail based on DMARC records. Yahoo is also the first ISP to publish a DMARC records with p=reject. This means that certain types of mail from Yahoo users are being rejected from ISPs checking DMARC ¹.

Certification Services

Yahoo does use Return Path Certified. Return Path Certified senders are given preferential treatment, delivering to the inbox with images enabled by default. Certified IPs also do not have the daily and hourly rate limits other senders do. Certified mail cannot guarantee inbox placement because user level preferences are always honored.
Yahoo was a relatively early adopter of Goodmail certification. In February 2010, however, they announced that they were no longer going to be guaranteeing that all Goodmail certified email will be delivered to the Inbox at Yahoo². Yahoo removed the Goodmail specific MX machines on or around March 24, 2010.

Support for Senders

Sender support is available at the Yahoo Postmaster Site. Yahoo has an extensive set of pages providing detailed delivery advice so senders can troubleshoot problems themselves.

Known Spamfilters

Yahoo does use the Spamhaus XBL and the Spamhaus PBL as part of their filtering strategy. No mail is accepted from any IP addresses on the blocklist until the IP is removed from the list.

Notes