As a user of our platform, you’re no doubt aware that we strive to make your sending experience as easy and painless as possible, so that you can focus on getting your mail to your customers and keeping them engaged. One of the ways we do this for you is not only collecting any bounces you may generate when sending mail, but also classifying those bounces into different categories, so that you or we can take action as appropriate.

Today we’re announcing plans to make a change to the classification of one particular bounce pattern. While we make changes and additions to our classification system all the time, most are minor, and so we usually do them without any formal notice given to our customers; the one we’re planning, however, will have a significant impact on many of our customers’ mailings, and so we thought it best to let you know in advance.

There is a bounce reply that we see frequently from Office 365(tm) hosted domains that looks like this one:

550 5.4.1 [xxxxxx@yyyyyy.zzz]: Recipient address rejected: Access denied []

We have historically believed this bounce reply to mean that the intended recipient of the message had configured some kind of personal filtering to reject messages from a sender; because of this belief, we have always classified this response with code 50, meaning that it’s a block bounce. Such a classification has never been 100% correct, because we usually see a high percentage of mail intended for the domain in question still getting delivered (so there’s no block in place), but we really don’t have a “personal filtering” bounce code. Moreover, we’ve done some research on the bounce and have now decided that our classification is entirely wrong and needs to be changed.

Our work to suss out the true meaning of this bounce reply has lead us to the conclusion that it should be classified as a code 10, meaning that it’s an indication that the address is invalid. This will change the bounce to one we call a “hard bounce”, and will also mean that the address will be suppressed from future mailings. This will mean that subsequent attempts by you to send to this address will be classified as “admin bounces”, with a code of 25.

For the average SparkPost customer, this change will mean a jump in the number of addresses on your suppression list, and a noticeable shift in some of your numbers, including fewer messages actually sent, lower block bounce rates, and higher hard and admin bounce rates. Because we think this change could be significant, we wanted to give you advance notice of it.

Obviously, any address that is suppressed will no longer receive mail from you, but then again, these bouncing addresses weren’t receiving your mail, anyway. However, should you receive any complaints from suppressed addresses about their not receiving your mail, we wanted to make sure that you understood the reason why they were suppressed.

Unless you hear otherwise from us (since we’ll be double and triple and quadruple checking our information to confirm our decision) we will make this change on April 18, 2018.

Thank you again for using SparkPost.