Postmaster

• What are some best practices when sending to Yahoo! Mail?
• Are there any guidelines for establishing SMTP connections to Yahoo! Mail?
• Does Yahoo! Mail offer a feedback loop program to help senders minimize complaint rates?
• Our hosted email accounts (e.g., for ISPs, universities, or other email hosting providers) are experiencing delivery delays when sending to Yahoo! Mail. What can we do?
• My email is being blocked by Yahoo! Mail. What can I do?
• As a sender, what can I do to ensure that my email goes to the right folder?
• Why was my SMTP connection rejected?
• Does Yahoo! use "greylisting" to reject messages?
• As a sender, how can I ensure uninterrupted SMTP access and prioritized delivery?

SMTP Error Codes

• What does a 4xx or 5xx error message indicate?
• Why am I getting “451 Message temporarily deferred” or "421 Message from x.x.x.x temporarily deferred" errors when sending mail to Yahoo!?
• 421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.55
• 421 4.7.0 [TS02] Messages from x.x.x.x temporarily deferred due to user complaints - 4.16.56
• 421 Message from (x.x.x.x) temporarily deferred - 4.16.50
• 421 Message temporarily deferred - [numeric code]
• 451 VS* Excessive unknown recipients - possible Open Relay
• 451 Resources temporarily not available - Please try again later [#4.16.5]
• 451 Message temporarily deferred - [numeric code]
• 553 5.7.1 [BL21] Connections not accepted from IP addresses on Spamhaus PBL
• 553 5.7.1 [BL22] Connections not accepted from IP addresses on Spamhaus SBL
• 553 5.7.1 [BL23] Connections not accepted from IP addresses on Spamhaus XBL
• 553 Mail from x.x.x.x not allowed
• 554 5.7.1 [MW01] Message content not accepted for policy reasons
• 554 Message type not allowed. UP Email not accepted for policy reasons
• 554 Message not allowed: DK Email not accepted for policy reasons

CertifiedEmail

• What’s the difference between DomainKeys verified mail and CertifiedEmail™?
• Why does a message appear as CertifiedEmail™ in the all-new Yahoo! Mail and not in Yahoo! Mail Classic?