Error: 554 Message not allowed - [numeric code]
You'll receive an SMTP error or failed delivery message that includes "554 Message not allowed" when the content of a message you're trying to send violates Yahoo Mail policies. Content that violates policies includes; objectionable links, malicious attachments, or bad header information.
Review the Yahoo Mail Terms of Service to learn more about Yahoo policies that can effect your ability to send messages to Yahoo Mail accounts.
If you're the administrator for the mail server in question, review your outgoing messages for objectionable content or practices.
Note: Yahoo is unable to disclose specific information regarding our filtering practices.