Posted by David Baker (Founder, CSO/COO)

“Bounce Handling and Controls”

If an e-mail address in your list ceases to be valid, you remove it from the list. There are more subtle aspects of bounce management that you might not be acutely aware of, however.

A bounce is a notification that your message, for whatever reason, didn’t make it to the recipient. Ideally, these bounces take the form of SMTP (define) codes, defined as a standard in RFC821. Using these codes, ISPs can communicate the reason for the bounce. Not everyone follows this standard, however, and accurate bounce handling may involve some keyword review of the replies.
Too Much Variability

Standard bounce code definitions were created in the early ’80s. Since then, the Internet and e-mail have undergone enormous changes. Messages now are refused for reasons that simply didn’t exist back then. This has led to systems administrators and software authors creating their own bounce definitions and explanations.
Result: Identifying and categorizing bounces is far from an exact science. A great variety exists in even the most common bounce types. “551 not our customer” is clearly equivalent to “user unknown.” As are “550 Sorry, no mailbox here by that name” and “550 is not a valid user.” Bounce processing must identify all these — and many more.

Misleading Bounces

Worse, many domains and ISPs don’t adhere to existing standards at all. In some cases, they deliberately mislead or are vague about the reason for refusing a message. What do “554 mail server permanently rejected message,” “550 service unavailable,” “550 administrative prohibition — unable to validate recipient,” and “550 unrouteable address” really mean? Are these permanent failures or only temporary?

Some will even return “user unknown” when they believe a message to be spam. They do this to discourage spammers from continuing to use the address.
Incorrect bounce messages can be returned due to misconfiguration or system errors. If a recipient database fails all delivery attempts, it may return “user unknown” even when it’s a temporary circumstance.

When faced with bounce messages they can’t identify, some sending systems simply ignore them or treat them as soft bounces. Clearly, this isn’t an ideal solution.
Failure to correctly process bounces wastes resources, both yours and the ISPs’ to which you send. Worse, it can result in blacklisting at individual ISPs, even on public blackhole lists.

Regardless of the bounce message’s exact wording, there are two types of bounces: hard and soft. Depending on whom you talk to, they might have more technical definitions; but here is the gist of what they mean.

A hard bounce means either the receiving server purposely rejected the message or the receiving server doesn’t exist. Examples of hard bounces are:

• The user doesn’t exist at the domain.
• The domain doesn’t exist.
• The message was rejected

A soft bounce typically denotes a temporary error with delivery and may be any response other than a hard bounce. Examples of soft bounces are:

• The e-mail server isn’t responding.
• The user’s mailbox is full.

Why Process Bounces?

It’s important to properly process bounces for a couple reasons. You don’t want to pay for e-mail messages sent to nonfunctioning addresses. If you don’t process bounces correctly, a mailing list’s natural churn will result in large portion of dead addresses on the list.

Monitoring bounces can help show a potential delivery problem. Perhaps an e-mail domain that represents a significant portion of your list has stopped responding. Perhaps your messages are being rejected. By monitoring bounces after every campaign, you can quickly correct any irregularities.

Most important, ISPs look at bounce information when determining whether they’re being targeted by a spammer. Spammers’ e-mail lists are of very poor quality. If an ISP detects a large percentage of invalid e-mail coming from one IP, the mail stream may be identified as spam and blocked.

Minimize Bounces

Here are some tips to help effectively deal with or minimize e-mail bounces:

  • ISPs recommend retrying hard bounces no more than three times. Depending on the quality of your list, opt in procedure and volume of emails being delivered on a regular basis that on hard bounce = a terminal address and should be flagged for re-validation.
  • Remove hard-bounced addresses from the list either immediately or after the retry attempt fails.
  • Remove soft-bounced addresses from the list if the address repeatedly generates bounces over a period of four to five e-mail campaigns. Take special note that some email environments have a higher instance of soft bounces and some have a higher instance of hard bounces (AOL, due to the changing of screennames).
  • Use a double or confirmed opt-in subscription process to minimize incorrect and false addresses from the start. Meaning, you should either have the user submit the email address twice for verification or have the user respond to their opt in request through either clicking through to the site or sending a reply email
  • Use an e-mail change of address service to help combat e-mail address churn in your mailing list. Note that this will typically only yield up to 10-20% valid addresses, so there is not a magic approach to re-validating bad email addresses.
  • And More….

  • Add an e-mail address update link to your e-mail and a profile update form to your Web site, enabling subscribers to update their address and preferences.
  • Consider contacting bounced subscribers via postal mail or phone (if you have contact information and permission) to obtain their new e-mail addresses.
  • To ensure subscribers enter their e-mail addresses correctly, include a script that checks for syntax errors upon submission.
  • Monitor bounce messages (particularly from key ISPs and domains) for signs of e-mail rejection. The message may have been rejected due to blocking or filtering, and you may need to contact the administrator of the receiving system. Depending on you internal capability to capture these codes and parse them or if you are using a third party system to deploy your emails, you need to find a way to delineate how to process and distinguish the two types of bounce categories
  • Monitor bounce rates continually, and establish a benchmark. Analyze the cause, and take appropriate action when a message lies outside of the norm. Though average bounce rates can vary dramatically, if your rate continually rises above 5 percent, you may have list input or hygiene issues.
  • Pretest messages for potential spam-oriented content to help minimize rejections by ISP and corporate spam filters.
  • The Solution Is Simple

    Some bounce reasons are unidentifiable. Those that can be determined can’t always be trusted. Given this, use repetition instead of categorization. It’s an extremely simple solution that doesn’t depend on categorizing bounces, so it isn’t prone to the above problems.

    If an e-mail address bounces consistently and repeatedly, consider it dead and stop mailing to it. It really doesn’t matter if the problem is deemed temporary or permanent. If the problem doesn’t go away, clearly it’s permanent.

    1. Bounce Handling Policy: senders should mark an address as “dead”, meaning the sender should remove the address from the delivery list and not attempt to deliver to the address until the sender has reason to believe that delivery rejection would not occur, if the following two conditions are both met:

    A. Three (3) consecutive delivery rejections have occurred; AND
    B. The time between the most recent consecutive delivery rejection and the initial consecutive delivery rejection is greater than fifteen days.

    A sender should have the capability to manage delivery rejections differently between ISPs, whether based on previous agreements or explicit requests from these ISPs.

    In other words, try the address three times over at least 15 days. If your messages are refused every time, treat the address as dead.

    Other considerations. There are two other issues related too list health that often don’t get talked about. SpamTraps , which is a big issue these days when old email addresses are recycled by the ISPs and serve as SpamTraps. Cleaning these out regularly won’t get caught by bounce handing, rather close monitoring of deliverabilty and close inspection of lists where there are issues (by domain).. Secondly, the issue of “mailbox” full is something that has grown the last few years. Most of the free accounts have higher storage limits than they did in the past, so seeing a mailbox full bounce code is rarre these days. But when it does happen it can often mean and “abandoned account”.

    While these are general guidelines to help inform, each company must adapt to their own lists, programs and issues. It helps to start with the right baseline understanding.

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