Soft bounces and rate limiting

What is your policy for handling soft bounces? What do you consider a soft bounce? What is the right thing to do about soft bounces?
The first step in talking about soft bounces is to define them. When I talk about soft bounces, I mean mail that has been rejected with a 4xx response during the SMTP transaction. As described by RFC5321, when a recipient MTA responds with a 4xx it is telling the sending MTA “Wait! I can’t take this mail right now. Come back a little later and try again.” The sending MTA will then continue to attempt to deliver the message until either it is delivered or until it hits the max delivery time, usually 3 – 5 days.
In a well behaved and RFC compliant MTA, messages that have reached the maximum time without delivery due to 4xx rejections will be converted to permanent rejections (5xx). With a correct MTA, this means too many emails in a row timing out shoud result in an email address being removed from future mailings.
For a number of reasons some ISPs, notably Yahoo, are using 4xx responses to slow down mail from some senders. Many senders treat this as a inconvenience and a frustration and try to figure out how to get around the rate limiting. The UK DMA published an article on soft bounces with the following words of wisdom.

Emails blocked in this way are recorded as soft bounces, so your soft bounces are important indicators of how ‘Spammy’ ISP’s think you are at a given point in time. They are an indication of a drop in reputation for the IP address you are using, uneven mailing patterns and/or an increase in the level of SPAM complaints your email is generating.  All of these things are things that you and your ESP can do something about!

This, in a nutshell, is why senders should not just grump about transient failures, but instead should look at their mailing practices. The ISPs using this technique to slow down senders typically are doing this on-the-fly, in that their limits are being put on mail that they see as problematic at that moment. In addition to the reasons given by the UK DMA, they are also monitoring complaints in real time and looking at the content of the message. If the ISP is seeing an increase in complaints about mail with a URL that is also in your mail, you may see rate limiting as a result. If the ISP is seeing an increase in complaints about mail from your IP, then you may see rate limiting as a result.
These are issues senders can address. You have the power and the technology to address the issue. Addressing the issue will improve your overall delivery.

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Reputation

Reputation is the buzzword in delivery these days. Everyone talks about building a good reputation and how to do it. Makes sense, the ISPs are always hammering on reputation and how critical reputation is. The more I talk with delivery folks on the ESP side of thing, the move I realize that there is a fundamental disconnect between what the ESPs mean when they say reputation and what the ISPs mean when they say reputation.
Many people handling delivery think that the bulk of reputation is wrapped up in complaint rates and bounce rates. I think they know the ISPs measure more than just complaints and bounces (spamtraps!) but really believe that most of developing a good reputation is all about keeping those complaints low.
This perspective may have been true in the past, but is becoming less true as time goes on. There are a lot of very smart people managing incoming mail at the ISPs and they are constantly looking for ways to better meet the desires of their customers. Lest we forget, their customers are not the senders, their customers are the end users. Their customers are not senders.
Part of meeting the needs of end users means actually giving them a way to provide feedback. AOL started the trend with the this-is-spam button, and other ISPs (ones that controlled the user interface at least) followed suit. For a very long time, reputation was dominated by complaint percentages, with modifiers for number of spamtrap addresses and number of non-existent users.
The problem is, these numbers were easy to game. Spammers could modify their metrics such that their email would end up in the inbox. In response, the ISPs started measuring things other than complaints, bounces and spamtraps. These other measurements are strong modifiers to complaints, such that mailers with what used to be acceptable complaint rates are seeing their mail end up bulked or even rejected.
Recently, AOL seems to have made some subtle modifications to their reputation scores. The result is mailers who have previously acceptable complaint rates are seeing delivery problems. When asked, AOL is only saying that it is a reputation issue. Lots of senders are trying to figure out what it is that is more important than complaints.
Tomorrow, I will talk about what I think AOL could be measuring.

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