Why is bounce processing not enough for successful e-mail campaigns?

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Bounce processing is a key component of overall list hygiene strategy. Doing it properly has a significant impact on your deliverability and overall sending reputation.

Traditionally, bounce processing was merely the rules you applied to e-mails generating either soft bounces (potential temporary failures) or hard bounces (permanent failures). These bounces are returned to servers, or perhaps an inbox, by the receiving server. Typically, “rules” state if an e-mail address hard bounces three times in two weeks, remove it from the list. A different set of rules is applied to soft bounces.

Unfortunately, these rules are not sufficient in today’s e-mail landscape and can lead to overcleaning of lists and missing major deliverability issues. A message returned as a bounce is one of the best indicators of how your e-mail is viewed by the outside world and gives you information to proactively manage it.

Most people will tell you that a soft bounce is typically an “out of office” response or other temporary problem. Hard bounces are most commonly believed to refer to a bad or expired e-mail address. Unfortunately, there is no rulebook for how ISPs’ and receivers’ servers return information to you. So, although the basic logic makes sense, it doesn’t work.

Here are some examples of error messages returned to an e-mail server. Note, these are all messages returned from a mailing sent to subscribed/opted-in users:

Soft/transient messages:

  • 450 4.7.1 <>: Recipient address rejected: Grey listing in use, please try again in 60 seconds
  • 450 4.7.1 Service unavailable; Client host [11.222.333.444] blocked using;
  • Unable to resolve DNS information for this domain
  • No more mail servers left to try.
Hard/permanent messages:
  • 550 Service unavailable; Client host [11.222.333.444] blocked using
  • 550 5.1.1 <> recipient rejected
  • Permanent failure resolving DNS information. Domain may be invalid or resource records are missing and/or invalid such that the domain cannot be resolved
  • 579 message content is not acceptable here
  • 554 5.7.1 The message from matches a profile the Internet community may consider spam.
  • 550 No such user—psmtp
Different ISPs communicate potential issues with the receiving domain, one as transient and one as permanent. Additionally, basing bounce processing on SMTP error codes returned can be problematic. In the examples, “550” is used to communicate an expired e-mail in one case and content being rejected as spam in another.

There’s a wealth of information about how receivers assess your messages within these logs. Yahoo communicates via soft bounce that an e-mail is not being delivered because you have hit a threshold for user complaints. Therefore, it’s time take a good look at your Yahoo customer activity. Another example is the “content rejection” type messaging shown in hard/permanent failures. A content review is necessary to figure out why filters are classifying it as spam.

If standard bounce processing had been applied in the examples provided, expired e-mails would have been removed; but, unfortunately, you would not know that the e-mail is actually being grey-listed because the content looks “spammy.”

Take the time to review server logs and the returned information. This will allow you to better understand your e-mail recipients, keep your list clean and create deliverable content—all key components for successful e-mail marketing.

Sarah Welcome is director of customer intelligence for International Data Group (

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