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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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