Soft bounces in email marketing can be a cause for concern. Read on for advice and tips to improve deliverability
  • Save

Many email marketers don’t see soft bounces as a major cause for concern and rarely have any specific strategies in place for dealing with them. However, soft bounces can be a strong indicator that one or more ISPs have queried the reputation of your IP address, mailing volumes, or the level of spam complaints generated, with the consequence that they have chosen to temporarily block your emails from getting through to customers.

Following the first blog post in our email marketing series which was on increasing open rates, in this piece we look at some of the common misconceptions about soft bounces and what can be done to improve deliverability.

The purest definition of a soft bounce is “a delivery failure with a 4XX/Transient bounce code” – examples of which are:

1. 421 Grey listing = Retry now and send again.
2. 451 no reverse DNS = Retry now and send again later after you fix your DNS.

But what about those misconceptions?

“A soft bounce just means the recipient’s
email account was temporarily unavailable.
Perhaps the server was busy, the recipient
was away, or their account was too full.”

It’s easy to make such excuses, but misguided notions such as this have marketers believing that the bounce had nothing to do with failings in their own marketing practices!

“What can I do to stop a server crashing
or a subscriber’s mailbox filling up
when they’re out of the office or away?’ 

Established guidelines might suggest removing the recipient from your mailing list after three bounces, but inbox sizes (including webmail accounts such as Gmail, Yahoo and Hotmail) have dramatically increased in recent years and, realistically, full mailboxes are unlikely to be the culprit.

The same is true of work email addresses: few companies start bouncing communications without letting the user know well in advance that their inbox is due a clearout. It is also unlikely that bounces are caused by overloaded ISPs or corporate servers as most companies will never allow themselves to be in that position in the first place.

In Azam’s email marketing team’s experience, the most likely cause of soft bounces is ISP blocking. The way ISPs deal with spam has become aggressive and sophisticated – the likes of Hotmail and Yahoo Mail are notoriously ruthless. They are quick to start blocking emails when they identify suspicious patterns of behaviour.

Common causes of ISP blocking are a lack of authentication; poor/unproven IP reputation; fluctuations in the volume of messages sent from your IP address and spam complaints attributed to certain addresses. Bear in mind that you may incur soft bounces because of the behaviour of companies sharing your IP address, making the way your ESP handles its shared IP addresses very important.

Emails blocked by ISPs are recorded as soft bounces, serving as an important indicator of how spam-orientated the ISPs think you are at that point in time. They may show 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 emails are generating.  All of these issues can be remedied by your Email Service Provider once identified.

Email marketing and how to efficiently grow deliverability rates
  • Save

Our email marketing series will continue with advice on minimising unsubscribes, optimising email creative for mobile handsets and more. Subscribe to so you are informed when the next article in the series is published.

Are you extracting the most juice from your email subscriber base? Write to results [at] or call +44 (0) 870 870 2222 to discuss how our award-winning eCRM and email marketing specialists can significantly grow your databases and enable you to maximise revenue from broadcasts to your subscribers. As an example, we are generating well over £100,000.00 in sales per weekly newsletter we create and deploy for a client! Contact us today to discuss your requirements.