Apparently, you will need to revise the email you are trying to send.
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 and Yahoo Messenger 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 obectionable content or practices
How did I find this information? I used my favorite search engine, Google, and entered this search phrase, using quotation marks as shown:
"Yahoo mail" "Error 554"
How hard was that?