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 email@example.com is not a valid user.” Bounce processing must identify all these — and many more.
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.
Here are some tips to help effectively deal with or minimize e-mail bounces:
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.