A 554 email error is sometimes thought of as a catch-all of email errors. It is generally used when there is a generic delivery failure that another email error code doesn't directly define a problem for.
Some mail servers also use a 554 error even if there is a specific error code that defines what went wrong all ready.
You should receive a bounce-back message from the server with a variation of email error 554 in the subject, and the body should contain your original message that you attempted to deliver.
Example bounce-back message
Delivery-date: Sat, 13 Oct 2012 04:40:30 -0700
Received: from mailnull by biz32.inmotionhosting.com with local (Exim 4.77)
for sender@YourDomain.com; Sat, 13 Oct 2012 04:40:30 -0700
From: Mail Delivery System <firstname.lastname@example.org>
Subject: Mail delivery failed: returning message to sender
Date: Sat, 13 Oct 2012 04:40:30 -0700
recipients. This is a permanent error. The following address(es) failed:
SMTP error from remote mail server after RCPT TO:<email@example.com>:
host mail.RemoteDomain.com [220.127.116.11]: 554 Invalid recipient
Received: from Admin by biz32.inmotionhosting.com with local (Exim 4.77)
for recipient@RemoteDomain.com; Sat, 13 Oct 2012 04:40:28 -0700
To: Recipient <firstname.lastname@example.org>
X-Loop: Recipient group <email@example.com>
From: "sender" <firstname.lastname@example.org>
Subject: Auto Reply: Out of Office
Date: Sat, 13 Oct 2012 04:40:28 -0700
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 RLY:B1
- 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)
Resolving a 554 email error bounceback
Because errors in the 554 series don't always bounceback for a common reason, they could be either a hard or soft bounce. To better understand soft and hard bounces, and general email bounceback errors you can read why does email bounce, bounceback, or error?
In our example bounce-back error above the reason the message has failed is because an auto-reply was trying to be sent to an account that didn't exist. So the RemoteDomain.com mail server responded back with a 554 Invalid recipient error.
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