AOL

AOL has set a very high standard for other ISPs in terms of both mail filtering and delivery support. They were the first ISP to provide clear diagnostic information in bounce messages, the first to provide senders with a feedback loop, and one of the first to offer whitelisting services.

Filtering decisions are based on a combination of sender reputation and email content. AOL users also have the ability to block mail from specific senders.

Delivering to
AOL does limit the amount of mail a sender can send based on the reputation of the sender.

Ratelimiting is accomplished in a number of ways, including returning a 4xx response during the SMTP transaction. They also limit the number of emails sent during a SMTP connection to 500 emails per connection. Each message is limited to 100 recipients per message. The connection limit is published on the AOL FAQ page.

Postmaster pages
AOL has a very good set of pages at their Postmaster Website. On these pages they provide information for senders and other ISPs.

Feedback loop
AOL provides a self administered FBL. Applications can be submitted on the FBL signup pages. To sign up AOL collects the following information:
 * Contact information for an individual responsible for the FBL
 * Corporate contact information
 * An email address for the FBL email
 * The IP addresses covered by the FBL

AOL has a 2 step process to confirm that the FBL applicant has the authority to receive complaints about mail from those IPs. The first step is to confirm that the FBL address is authorized to receive the email. They do this by sending an email to abuse@ or postmaster@ the domain in the FBL example. For example,

Once AOL have confirmed that the email address involved is authorized to collect FBL emails by the domain owner, they need to confirm that the domain is related to the IP address. They are looking for a clear link between the IP addresses the FBL is for and the FBL email address. Generally, the easiest way to pass the test is to have one of the email addresses in the whois record point to the same domain as the FBL email address. However, there are a number of ways to meet the AOL standards for IP Ownership.

AOL sends FBL email in ARF format. AOL does redact the recipient's email address. Senders are encouraged to encode the recipient address in the headers, either through the use of VERP or a custom header, so that they can stop sending mail to those recipients who hit the this is spam button.

Whitelist
Whitelist Signup

The AOL Whitelist is open to senders who have a low complaint rate. Personal experience suggests that complaint rates higher than 0.6 or 0.7% preclude senders from being admitted to the AOL whitelist program.

AOL also has an Enhanced Whitelist. This is a whitelist that means mail is in the inbox and all images are loaded. There is no way to apply for the enhanced whitelist, only the best senders get put on the EWL. What does best sender mean? AOL will not reveal the methods for determining who is on the EWL but very low complaints and low unknown users is important. Personal experience suggests that complaint rates significantly below 0.1% are required for inclusion on the EWL. In November 2009, AOL changed the criteria for inclusion on the EWL. Now, rather than just looking at complaint rates, AOL will also be looking at the reputation of an IP address. There is no way to apply for this whitelist. Inclusion is automated.

Certification Services
AOL does use Goodmail CertifiedEmail for inbox placement. Senders certified by Goodmail have mail delivered to the inbox with links and images enabled.

Support for Senders
AOL provides web based support for senders who are having problems delivering to AOL. The response time varies depending on how swamped the AOL postmaster team is. It can take a week to get a response from the postmaster pages. While you are waiting for a response, you can work through your own internal troubleshooting and attempt to solve the issue. Unless the problem is a permanent hard block like HVU:B2, improving your stats should resolve the blocking issue.

A word of advice, use the correct form whenever possible. The "other requests" page can have much longer delay than the requests for help on specific issues.

AOL support pages

AOL Postmaster Blog

AOL Articles at Word to the Wise blog

Known spamfilters
Most of AOL's spamfilters are built in-house and they do not use commercial spam filtering services in general. The one exception is that AOL does use a commercially available list of dynamic IP addresses.