This article provides general guidelines for postmasters who plan to send email to Yahoo/gmail customers.
The mission of any ESP is to deliver messages that users want to receive, and filter out messages they don't want. The simplest way to ensure that your messages are delivered, then, is to avoid looking like a spammer. Below are a few suggestions on how to do so.
Send email only to those that want it. Spammers write to many people who don't want their mail, so our anti-spam filters are designed to identify that behavior. To avoid being perceived as a spammer, use an opt-in method of subscription for your mailing list. Better yet, utilize a confirmed opt-in process where subscribers actively verify their intent to receive your mailings by clicking on a confirmation email before being added to your list.
Use email authentication such as DKIM. This will help us identify that the email is legitimately from you and, if you sign all your email, it will help us identify forgeries, too. In addition, using dedicated domains for different mail streams (e.g., transactional messages vs. marketing emails) is also a recommended practice. DKIM.org is a good resource for information about DKIM.
Segregate your mail streams by IP addresses and/or domains since Yahoo! Mail keeps reputation data about each IP address and domain you send from. For example, by segmenting the mail stream that delivers marketing mail from another that sends transactional messages, you make it easier for Yahoo! Mail to determine and assign each mail stream's reputation.
Consistently manage your lists by paying attention to hard and soft bounces as well as inactive recipients. Persistent emails to these addresses are a surefire way to get your connections deferred. If your messages are being blocked, look closely at any SMTP Reply Codes our mail servers are returning to you and be sure you're addressing the problem.
Enroll in our feedback loop. If you are signing your emails with DKIM or DomainKeys, our Complaint Feedback Loop program can help you track your spam complaint rates and keep them in check.
Use a consistent "From:" header address. Your domain name is an important element of your brand. Using it consistently helps us and your recipients to distinguish your email from spam. Additionally, using a static From: address helps users who have set up filters to route messages to a specific folder.
Honor unsubscribe requests as fast as you honor subscribe requests. When a user unsubscribes, they don't want to receive that mail anymore. Promptly removing them from the list should help prevent users from marking your messages as spam in the future.
Be CAN-SPAM compliant. Regardless of where in the world you're sending your mail, make sure that you adhere to the requirements stipulated by the CAN-SPAM Act.
Publish reverse DNS (PTR) records for your sending IPs. If there is no reverse DNS entry for your IP address, or if it looks like a dynamically-assigned IP instead of a static mail server, Yahoo! is more likely to downgrade its sending reputation.
Secure your mail servers. Ensure your mail servers are not open to abuse. If your servers act as "open proxies" or "relays," spammers may attempt to send their own mail from your systems. Keep your software up to date with the latest security patches, and always filter user-generated content before sending it out, to help prevent spammers from using your resources and tarnishing your reputation.
Use common-sense settings. While we have not published guidelines for numbers of connections you can concurrently use, we ask that you treat our resources with respect. The more you take, the fewer there are for others, which may force us to defer your connections.