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Email Delivery SMTP Error Codes
Posted by: Poster Avatar Susan Farrell @susan_a_farrell    |    02/06/2017 08:00 AM    |    Categorized under:  Email MarketingMarketing Automation

OK, before we get started, let’s make sure we are all using the same definitions here (you can skip this part and go straight to the infographic, if you like).


Q.  What is email deliverability and how is it measured?
A.  Email deliverability rate is the number of emails that you sent that were successfully delivered to recipients. It is not a reflection of whether your email went into the spam or junk folder of your recipient.

Q.  When is an email considered to be a successful deliverable to a recipient?
A.  The receiving email server acknowledges that the email address is valid on their domain and is therefore "delivered” to the email recipient.

Q.  When is an email considered to be a failed deliverable to a recipient?
A.  The receiving email server replies with an error code that the email address does not exist.  Here are some examples of failed email reporting:

    • 550 5.1.1 User Unknown In response to the RCPT TO command.Could not connect to server. Type=A.
    • 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup
    • 550 Requested action not taken: mailbox unavailable In response to the RCPT TO command.Name service error for Type=A: Host not found.
Q.  What are some of the most common reasons why an email address does not exist?
A.  The most common reasons that our support team sees are:

    • The addressee is no longer with the company (domain) that their email was associated.
    • The company domain has changed; for example the domain was and now it is
    • A list of procured names was uploaded into the CRM. Unauthorized email blasts (addressed to more than 10 names) are considered spam.
Email Delivery Error Codes and What to Do
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