551 email error is normally ecountered when there is an attempted delivery to a mail server that doesn't handle mail for the specified recipient.

You should receive a bounce-back message from the server with a variation of email error 551 in the subject, and the body should contain your original message that you attempted to deliver.

Example bounce-back message

Return-path: <>
Envelope-to: sender@YourDomain.com
Delivery-date: Thu, 25 Oct 2012 11:19:59 -0700
Received: from mailnull by biz100.inmotionhosting.com with local (Exim 4.77)
id 1TRS2B-00033h-Fn
for sender@YourDomain.com; Thu, 25 Oct 2012 11:19:59 -0700
X-Failed-Recipients: recipient@RemoteDomain.com
Auto-Submitted: auto-replied
From: Mail Delivery System <Mailer-Daemon@biz100.inmotionhosting.com>
To: sender@YourDomain.com
Subject: Mail delivery failed: returning message to sender
Message-Id: <E1TRS2B-00033h-Fn@biz100.inmotionhosting.com>
Date: Thu, 25 Oct 2012 11:19:59 -0700

This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
 
recipient@RemoteDomain.com
SMTP error from remote mail server after RCPT TO:<recipient@RemoteDomain.com>:
host RemoteDomain.com [123.123.123.123]:
551 not our customer
 
------ This is a copy of the message, including all the headers. ------
 
Return-path: <sender@YourDomain.com>
Received: from bell.ca ([123.123.123]:42593 helo=Admin)
by biz100.inmotionhosting.com with esmtpa (Exim 4.77)
(envelope-from <sender@YourDomain.com>)
id 1TRS1q-0002fq-S4; Thu, 25 Oct 2012 11:19:39 -0700
Date: Fri, 26 Oct 2012 00:03:21 +0600
From: Sender <sender@YourDomain.com>
MIME-Version: 1.0
To: <recipient@RemoteDomain.com>
Subject: Meeting proposal

Common variations of the 551 email error

  • 551 5.1.1 recipient is not exist
  • 551 5.1.1 user does not exist
  • 551 Denied IVF

  • 551 not our customer
  • 551 recipient@RemoteDomain.com failed to route the address
  • 551 <recipient@RemoteDomain.com>... User not locali
  • 551 Relay request denied
  • 551 This is not a relay host - mail must be to or from host domain.
  • 551 Unknown user.
  • 551 User not local. Authentication required for relay
  • 551 User not local; please try <forward-path>

Resolving a 551 email error bounceback

The 551 series of errors would typically be a hard bounce. For an in-depth review on the differences between soft and hard bounces, and general email bounceback reasons 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 the server for RemoteDomains.com doesn't control mail for the recipient@RemoteDomain.com e-mail address. 

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