• Monitor hard and soft bounces as well as inactive recipients. Persistent emails to these addresses will get your connections deferred.
• Consider sending a reconfirmation email to inactive subscribers periodically. Or just remove them entirely.
• Sending email to users who are not reading them, or who mark them as "spam," will hurt your delivery metrics and reputation.

• Use and honor an opt-in method of subscription for your mailing list. Make sure subscribers have actively verified their intent to receive your mailings.
• Honor the frequency of the list's intent. Don't start sending daily emails to subscribers of your monthly mailing.
• Honor unsubscribe requests quickly.

• Test your emails' visual look with image placeholders. Many users won't see images in your email by default.
• Link to domains, not IP addresses.
• Use standard ports.
• Don't include: HTML forms, , or embedded objects (like flash or ActiveX.)

Have separate IPs and streams depending on your email content type. 

• Shared email services can have multiple domains using one IP to send email. 
• If the same IPs are sending unsolicited commercial email, and your presumably valid email, then your reputation can be impacted. 

• Update your software with the latest security patches.
• Filter user-generated content before sending it, to prevent spammers from using your resources.
• If your servers act as "open proxies" or "open relays," spammers may attempt to send their own mail from your systems.
• At a minimum, your SMTP servers should identify the originating IP addresses that of the email and indicate this in the email headers to help you diagnose spam problems.

If your messages are being blocked, look closely at any SMTP error codes our mail servers are returning and make sure you're addressing the problem:

• Don't retry 5xx messages - Retrying permanent errors increases the chances your mail will be deprioritized.
• Retry 4xx messages - This is a temporary error.
• Enroll in our feedback loop - If you're signing your emails with DKIM or DomainKeys, our Complaint Feedback Loop program can help you track and manage your spam complaint rates.

• Check if you're DKIM signing the email. DKIM signature helps Yahoo authenticate that email is safe, secure and from the senders who claim to send it.
• Not being DKIM signed makes your emails appear to be sent by a spammer.
• See DKIM.org for information about DKIM.

Yahoo is more likely to downgrade an IP’s sending reputation if:

• There's no reverse DNS entry for your IP address.
• Your mailing IP looks like a dynamically-assigned IP instead of a static mail server.

• Regardless of where in the world you're sending your mail, make sure you adhere to the requirements stipulated by the CAN-SPAM Act.
• Invalid DMARC records can cause problems if you're sending from non-compliant bulk mailing services.