A 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
Delivery-date: Thu, 25 Oct 2012 11:19:59 -0700
Received: from mailnull by biz100.inmotionhosting.com with local (Exim 4.77)
for sender@YourDomain.com; Thu, 25 Oct 2012 11:19:59 -0700
From: Mail Delivery System <Mailer-Daemon@biz100.inmotionhosting.com>
Subject: Mail delivery failed: returning message to sender
Date: Thu, 25 Oct 2012 11:19:59 -0700
recipients. This is a permanent error. The following address(es) failed:
SMTP error from remote mail server after RCPT TO:<recipient@RemoteDomain.com>:
host RemoteDomain.com [184.108.40.206]:
551 not our customer
Received: from bell.ca ([123.123.123]:42593 helo=Admin)
by biz100.inmotionhosting.com with esmtpa (Exim 4.77)
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>
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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