Although AT&T uses the Yahoo! infrastructure, once email makes it through the AT&T gateway, your email is still subject to analysis and may experience delivery problems. Most of the time, your email performance at AT&T should be identical to Yahoo!. If you are having delivery problems at Yahoo!, you will see identical delivery problems at AT&T, Bellsouth and SBC. On rare occasions, delivery to AT&T may be slightly different than at Yahoo!, and that may be due to not following AT&T’s best practices or being on a blacklist.
When experiencing deliverability issues at AT&T, be sure you are following the best practices for both AT&T and Yahoo!. Some deliverability issues can be resolved with a quick review and implementation of best practices.
- Determine if your IP addresses are on Blacklists.
AT&T uses the following blacklists in their filtering decisions. If your IP address is listed, refer to the Blacklist listing criteria and fix the cause.
- Check your SMTP logs for clues regarding the issue.
AT&T blocks email with its own realtime block list. If your email is being throttled or blocked, check your SMTP logs for a response indicating a possible cause.
- Determine if your email authentication is working properly. A misconfigured SPF, DKIM or DMARC record can result in email being filtered or blocked.
- Check your connection and throughput settings to see if you are sending too much email to AT&T servers. Reduce the settings below suggested values if throttling occurs. The suggested settings are:
- 5 simultaneous connections per sending IP address
- 100 recipients per connection
- Check your retry settings and increase time between retries if still experiencing throttling issues. You may want to limit the number of retries per message to 2 or 3 temporarily if needed until you can isolate a cause of the deliverability problems.
- Determine if you sent email from more than one mail-from (aka. Mfrom or Return-path) address within a single SMTP connection. Do not send e-mail where your sending identity changes often when attempting to connect to AT&T mail servers or you will be blocked. AT&T only allows a maximum of 1 mail-from domain change per session.
- Check your header information. If you are sending messages to AT&T without proper header information that does not show at least 1 MTA hop, you will be blocked. AT&T suggests that you connect properly to their servers with at least 1 MTA hop.
- Determine if you are trying to verify AT&T email addresses without sending an email. Each time AT&T mail servers detect an attempt to verify if a email address exists, by connecting to their mail servers, but not transmitting a message, you will be blocked. Attempts like this appear to AT&T as a Dictionary/Harvesting attack.
- Determine if you are sending bulk email from a home computer. If you send bulk email directly to AT&T from a home computer, you look like one of the “zombie” spam machines that are responsible for much of the spam and phishing email on the Internet. All major email services, including AT&T, regularly block email from these sources (usually dynamic IPs).
- Check your email to ensure it is not being perceived as a phishing email.
- In HTML messages, make sure all the links are transparent. Discrepancies between the apparent URL and the “real” URL of the destination site will trigger anti-phishing warnings. Don't use HTML code to make it look like the destination is a part of your domain.
- If sending using a subdomain, use a real subdomain name: http://customers.businessname.com. Avoid using URLs that do not resemble real subdomains: http://www.businessnamecustomers.com.
- Avoid using IP addresses in the URL: http://0.0.0.0. Use a real domain name.