Understanding Email Delivery Failure Causes

There are several places in which email delivery to a recipient's inbox can fail once a message leaves our system:

Delivery failures at each of these possible locations can occur for many different reasons. Here are the most common reasons for failures at each location:

1. Connection blocked

A connection problem occurs when our outbound email servers are unable to initiate or maintain a connection in order to deliver email messages. Connection problems can be caused by any of the following: 

  • Overloaded servers at the receiving ISP. In our experience, free web-based email providers are the most likely to experience server overloading which impact connections.
  • Dynamic delivery blocking resulting from complaints received from email recipients. A dynamic delivery block is a direct result of feedback that ISPs receive from their recipients through the "Report Spam" button. These blocks usually last from hours to days. Many major ISPs use this strategy to limit spam from certain vendors.
  • Dynamic delivery blocking resulting from sending email to "trap addresses". Many major ISPs and spam monitoring organizations set up special "secret" email addresses, and if mail is received at these addresses, it is known to be unsolicited, and the email sender's deliveries can be temporarily blocked across the entire ISP as a result.
  • Permanent delivery blocking. ISPs may set up permanent connection blocks from certain senders based on the sender's reputation and delivery history, both with regard to complaints, and also proper removal of bounced addresses. our enforces its Acceptable Use Policy, and automates bounced email address removal in order to ensure that permanent delivery blocks do not occur. 

2. Bounced (or "returned") emails

An email is bounced when our email server is able to connect to the receiving server, but the receiving server rejects the incoming email message. An email message can be rejected for several reasons, the most common of which are:

  • The email address does not exist (a "permanent" or "hard" bounce)
  • The destination mail box is full (a "temporary" or "soft" bounce)

our system automatically distinguishes between "hard" and "soft" bounces, removing "hard" bounces immediately, and removing "soft" bounces only after a threshold of "soft" bounces has been reached for a given address.

3. Filtered and discarded

4. Filtered to junk email folder

Many receiving mail servers and clients employ spam filters which either silently discard email messages without any notification to the sender, or place email messages in a "bulk" or "junk" folder where they are unlikely to be seen by the receipient. Several factors influence how these filters work, including: 

  • The content of the message
  • The number of complaints that a given message or sender generates from recipients of a given ISP
  • The reputation of the message sender, as reported by third-party spam monitoring organizaitons
  • Whether or not the message uses email authentication (such as SPF/Sender ID and DomainKeys/DKIM)
  • Emailing to "spam trap" (or "probe") email addresses

Additionally, newer email clients are giving recipients more control over filtering, making it easier for recipients to configure filters based on individual senders, message content, or other information.


The above list illustrates the importance of using commonly-accepted best practices for list building and deploying email campaigns to ensure the highest levels of deliverability. A 100% opt-in list that is built in-house, combined with proper list management practices and quality, relevant content are the best ways to ensure the highest levels of deliverability.

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