Most email marketers do not view soft bounces as being a major cause for concern, so rarely, if ever, have any specific strategies been in place for dealing with them. Soft bounces are a very strong indication that one or more ISPs were concerned enough about the reputation of the IP address you are using,  your mailing volumes or the level of SPAM complaints you are generating to temporarily block your campaign.

Over the last two years our agency has begun to take soft bounces very seriously indeed. Compared with soft bounces, hard bounces are relatively straight forward - there are far fewer reasons for their occurrence and only one way of dealing with them, which is to remove them from your list.

The reason so many people don’t treat soft bounces seriously enough seems to be because most definitions of a soft bounce are outdated.  

Here are but two typical examples of the definition of a soft bounce taken from well respected sources:

• A soft bounce just means the recipient's email account was 'temporarily unavailable'. Maybe their server was busy, or the recipient was away on vacation, or their account was too full.
• A soft bounce occurs when the recipient's mail server replies with an error other than 5xx, or never replies at all. An example of a soft bounce error could be caused by a server that overloaded or a user whose mailbox is full.

Most current definitions of a soft bounce do not really convey any sense of urgency or importance, worst of all they seem to indicate that the bounce had nothing to do with your mail marketing practice at all. After all what can you do to stop someone’s server crashing or being taken down for maintenance or a subscriber’s mailbox filling up while they are on holiday?

Some ESPs set the default for removing soft bounces at 10 or more and best practice guidelines typically suggest removal after 3 subsequent bounces, further reducing the sense of urgency.

General definitions of a soft bounce and guidelines for dealing with them haven’t changed for at least five years!

In this time inbox sizes have dramatically increased so 'mailbox full' messages are highly unlikely to occur; Gmail currently offer 7GB of storage! I don’t know anyone who has exceeded their Gmail or for that matter Yahoo or Hotmail storage limit and even if there are such people, they are hardly representative of the average consumer. Even if you have a large number of people using their work email address, how many companies start bouncing potentially valuable communications without letting the user know that their inbox is due a spring clean.

I also suspect that bounces caused by overloaded ISPs (Hotmail etc.) or corporate servers are not as common as the definitions quoted above would seem to indicate.

This means that the most likely cause of soft bounces is ISP blocking.

The way ISPs deal with spam has become much more aggressive and incredibly sophisticated. For a start they are very quick to start blocking emails when they identify suspicious patterns of behaviour from a given sender, IP address or range.

Typical causes of blocking are: lack of authentication or accreditation; poor or unproven IP reputation; fluctuations in the volume of messages sent from the IP address you are using and spam complaints attributed to your IP address. It's important to 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 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!

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

• 421 Grey listing = Re-try now and send again
• 451 no reverse DNS = Re-try now and send again later after you fix your DNS

Soft bounces should be regularly investigated because we believe that they are more likely to be caused by temporary ISP blocking than temporary problems related to individual subscribers.

I would strongly recommend that anyone wanting to get a better understanding of where soft bounces fit in the overall scheme of deliverability should read the excellent 4 part series of articles on 'How Email Works' by Dennis Dayman of Eloqua

Dela Quist
Alchemy Worx

This article was written by Dela Quist CEO of Alchemy Worx and is based on an article in the current issue of Email-Worx a fortnightly newsletter for people interested in email marketing issues.