Common variations of the 554 email error
554 5.7.0 Message Size Violation
554 5.7.1 Message blocked due to spam content in the message
554 5.7.1 [P4] Message blocked due to spam content in the message.
554 5.7.1 <email@example.com>: Relay access denied
554 5.7.9 Message not accepted for policy reasons. See http://postmaster.yahoo.com/errors/postmaster-28.html
554 delivery error: dd This user doesn’t have a ymail.com account
554 Denied (Mode: normal)
554 Invalid recipient
554 Malformed mail denied!
554 Message is not RFC compliant; missing “Date” header
554 Message not allowed – [PH01] Email not accepted for policy reasons. Please visit http://postmaster.yahoo.com/errors/postmaster-27.html 
554 Message permanently rejected
554 Message refused
554 no valid recipients, bye
554 rejected due to spam content
554 rejected due to virus
554 <firstname.lastname@example.org>: Relay access denied
554 Sorry, no mailbox here by that name.
554 Spam detected
554 Spam violation, the content of your e-mail contains illegal characters, re-sent after examination
554 Transaction Failed Spam Message not queued.554 Virus found, message permanently rejected (#5.3.0)