Receiving "554 Message not allowed - " when sending email to Yahoo!
This article describes why you are receiving an SMTP error or bounce message containing the text string "554 Message not allowed - "
This error message indicates that your email was not accepted because the mail in question may contain characteristics that Yahoo! Mail will not accept for policy reasons. For instance, it is against Yahoo! Mail's policy to accept messages with malicious content or manipulated header information, such as a falsified origination point, altered sending date, or forged email address.
If you are the administrator for the message content or mailing policies, we ask that you review outgoing messages for potentially objectionable content or practices. Please know that Yahoo! is unable to disclose specific information regarding our filtering practices; however, we do publish recommended guidelines at this page for review.
We thank you for your understanding in this matter.