The Big Bounce

Planning an email campaign?  It can be as easy as 1, 2, 3:  write the copy, check the content, test the message and send.  With high expectations, you anticipate that the majority, if not all of your campaign emails, will be delivered without a fuss.  However, you do expect to see a few bounced back addresses from your sender list, among the clickthroughs, purchases and refer-a-friend links, which are typically reported by the live delivery analysis provided by your ESP.

Most senders will retain bounced back addresses to resend to them in the future, believing these bounces are likely the result of full mailboxes or misspellings.  The bounce report, depending on the technological sophistication used by your ESP, may divide these returns into categories.  A complete detailed report may allow the sender to distinguish the difference between the real reasons for the bounce from a full mailbox.  You might question the necessity of such detail – Just what are bounce categories and how do they affect your email campaigns?

The Bounce Categories

Email bounces can be either synchronous or asynchronous: the difference between them being the measured length of time for the bounce message to return to the sender.  Synchronous bounces are instantaneous as the failed delivery attempt is bounced back immediately, where an asynchronous bounce comes in awhile after the message was sent out.

The speed in which an email is returned in a bounced state is the result on how the message is intercepted.

  1. An email sent to an invalid address for example, will be terminated when the receiving server identifies the address as invalid, the connection is cut and the email is returned: a synchronous bounce.
  2. An asynchronous email bounce occurs when the receiving server attempts to process the invalid email, acknowledges the message and continues to attempt to deliver it until the delivery fails.

After the email bounce is determined to be synchronous or asynchronous, it is assigned to one of two categories using standard RFC codes, which can be identified in the return email header.

The Soft Bounce

A soft bounce is email that has bounced back to the sender, undelivered after it has been accepted by the recipient’s mail server.  This is usually a short-term condition with an expectation of clearing up in the future.  However, it is good practice to monitor soft bounces and remove them from your list when they bounce a certain number of times in a row.  Advanced email tools automatically handle this process.

Soft Bounces may occur when

  1. An Email is returned undelivered because the receiver’s mailbox is full at that time.
  2. An Email Message Size is too large to be delivered.
  3. Auto responders, such as a vacation out of the office message may be incorrectly reported as a bounce.

The Hard Bounce

All things not being equal a Soft Bounce on one sever maybe interpreted as a Hard Bounce on another server.  A hard bounce normally perceived as a long-term or permanent condition is generally not expected to clear up any time soon.  It is good practice to remove hard bounces when they occur however, you might want to develop an internal policy to remove the address after a few consecutive bounces as hard bounces may clear up when temporary condition occurs such as a temporary system fault or a blacklisted domain.

Hard Bounces may occur when

  1. Recipient address is misspelled.
  2. User doesn’t exist
  3. Your domain is blacklisted

Some ESP’s such as Dundee Internet offers a more granular bounce grouping rather than just a “Hard and Soft Bounce” report, accentuated with a uniquely colorful graph for easy evaluation.

The following nomenclature is used by  ListManager ™

Technical failures: user specific disk space problems.

Uncategorized failures: A failure that doesn’t easily fit into one of the other categories.

Invalid Users: For some reason, the email addresses are not legitimate ones (this is often caused by spelling errors).


Content Blocks: Content blocks are responses that indicate that the message is ejected for issues detected by a content filter. The filter may conclude that your message contains spam-like content, non-RFC compliant content, blacklisted URLs in the content,etc

MailStream blocks: You can learn more about MailStream blocks here.

End User complaints: Spam complaints made about the mailing.

Recipients remaining: The number of recipients for a mailing that are current pending in the Active Recipients queue. These users may not have been attempted yet, could be in retry due to transient delivery failures, or could be inactive due to a block by the destination IS

Next Up:  How to actively manage your bounces.

*** invalid addresses being caused by typos…if the addresses were validated in a closed loop confirmation system, invalid addresses because of typos would virtually disappear.