Difference Between Hard and Soft Bounce

“Your email failed to be delivered”. Again. Why? Even when you think you configured everything properly, there are various reasons why your email might bounce. Some might be really your fault but most of the time, it’s things out of your control that cause emails to come back in a matter of seconds. Let’s analyze each case one by one and talk about the difference between a hard bounce and a soft bounce.

Hard bounce vs soft bounce

Whenever your email bounces, you’ll receive an automatic email right away that will look something like this:

Oftentimes, an error message clearly indicates what the reason was for a bounce along with advice on how to fix it. Sometimes, though, the message is not so clear and you need to know how to interpret it.

In general, we differentiate two types of email bounces – soft and hard ones.

Hard bounce

A hard bounce is a permanent failure of email delivery. It indicates that there will be no further attempt to deliver a message and you need to find a different way to reach the recipient.

An email usually hard bounces when the email address you typed in doesn’t exist. Alternatively, a recipient’s ISP (Internet Service Provider) can just reject your email for numerous reasons.

Soft bounce

A soft bounce is a temporary failure of email delivery. Although an email failed to deliver this time, often a server will make further attempts a bit later. Sometimes all you need to do is wait.

An email bounces softly when the recipient’s mailbox is full, either side hits some limits or a message is deemed too large to be accepted. 

Notifications related to an out-of-office or other autoresponders are also sometimes treated as soft bounces but they work a bit differently. More about those below.

Some emails bounce for some other reasons and the way clients communicate these errors is sometimes far from obvious. We’ll discuss those as well.

Causes of hard bounces

Hard bounces happen for many different reasons. Here are the most common ones in more detail:

The recipient does not exist

This error indicates that the email address you used doesn’t exist. A simple typo could cause it but maybe a nonexistent domain is the reason here. Or maybe someone doesn’t work at a given company anymore and their email address was removed – this would also cause a hard bounce.

When an email address you used doesn’t exist, you’ll often receive a 550 5.5.1 server error, as was the case above. If the reason behind rejection is unclear but you see these mysterious numbers, you can quickly connect the dots and react accordingly. We covered this error in more detail in another article on our blog.

If this is the reason for the bounce, double-check if you didn’t mistype your recipient’s address. Often times, however, there’s nothing you can do other than removing a recipient from the list. Keeping such contacts on your email list and having your emails bounce over and over again will only affect your deliverability. Negatively.

Your email was blocked by a server

Modern email servers reject tons of emails they suspect of being unwanted. And while they’re almost always right to do so, sometimes they get rid of perfectly legitimate t emails without a moment of hesitation.

The reasons for this can be many. Often, a lack of proper authentication plays a role so make sure you have SPF, DKIM as well as DMARC set up. Check also if you have PTR Records set up and if they match A record from your domain’s DNS.

To improve the experience of its users, ISPs also look at their past behavior when determining whether an email should be accepted or not. If you keep sending emails but recipients never care to open them, a server may finally reject them before they reach an inbox to save them the trouble. For that reason, it’s good to clear your mailing lists every now and manually remove inactive contacts.

If none of these helps and your emails still bounce, it’s worth seeking support. If you experience problems with a particular domain, reaching out to them directly might get you whitelisted for the future deliveries. Your ESP may be able to both advise and solve some problems on your behalf.

Mailbox is full

Each mailbox comes with a certain quota of data dedicated to outgoing and incoming emails. When the limit is reached, each new message will likely bounce once and for all. Some providers might give a delivery another try a few days later in which case this would fall into a soft bounce category.

But don’t hope for that. If you’re getting such alerts, try reaching the recipients some other way. Maybe they’re not even aware of this fact. In many cases, unfortunately, this might indicate an abandoned account so you’re better off searching for a different contact method anyway.

Challenge-response error

This error is of a bit different type than its predecessors on our list. Some people set up an additional firewall meant to authenticate senders. Sometimes email providers set those up by default too. 

When you email such contact for the first time, you’ll get an automatic response. Usually, you’ll be asked to answer a question or perform some action to verify that you’re a legitimate sender. Once you do, your email will be delivered. If you, however, ignore this email and a few days pass, your email will bounce.

The only thing you can do to avoid it is to, well, complete the challenge. The good news is that in most cases once you prove you’re a homo sapiens, you won’t have to repeat this boring routine again.

Causes of soft bounces

Soft bounces also happen every now and then. Some are harmless, some indicate just temporary problems but many might foreshadow imminent hard bounces. Here are the most common motives behind soft bounces:

DNS Failure or other errors

DNS Failure is a common error caused by the receiving side. It indicates that there was some issue with Domain Name System and a message couldn’t be delivered at the time. Likely, a server was temporarily down or was misconfigured. If that was the case, a delivery will likely be retried over the next days and only if it fails multiple times, a hard bounce will occur.

Sometimes, this error might also appear if the domain you send to doesn’t exist. In such a case, there will be no future attempts and you’ll need to look for other ways to contact a recipient.

Any other errors can also occur during a transmission – for example, a connection may time out. In such a case, a server is likely to retry several times at a later time before it gives up.

A message is too large

Just as mailboxes have capacity limits, also individual messages are often subject to limitations. And while your outgoing servers might allow some messages with large attachments, it might not necessarily be true for ISPs.

If a message cannot be delivered due to its size, many ESPs will retry sending it over a specific time period – usually several days. If they still fail to deliver by that time, they’ll be permanently rejected.

A limit is reached

Some ISPs might also impose limits on the number of emails that can be received, to prevent flooding inboxes with spam. While it’s fairly rare for legitimate messages to be stopped like this, it might happen. In such cases, delivery will likely be retried several times, and only if none of the attempts succeed will the email bounce.

Autoresponder is triggered

Very often, sending to a large group of recipients will result in a wave of autoresponders hitting your inbox. This is especially true during the holiday season. Very often these messages are merely informing you that the recipient is currently out of the office and will get to your message a bit later. In such cases, the message has arrived in their inbox anyway so there’s nothing you have to do. It won’t hurt to follow-up a few days after they’re back, though, if you don’t hear from them until then.

It’s worth reading through such autoresponders, however. Sometimes they may inform you that a person you’re writing on moved on to a different company and no longer reads these messages. Companies tend to keep such mailboxes alive and leave instructions on who to contact instead. If that’s the case, make sure you update the contacts on your mailing list.

Wrapping up

We hope the difference between both types of bounces is more clear now. You should also have an idea on how to mitigate the risk of seeing them for your next campaign. If you’re interested in more tips on improving your emails, check out other articles on our blog. We talk about email sending, testing, and everything that revolves around email functionalities.

Also, check out Mailtrap – an environment for testing your emails on staging and development. Create an account for free and never spam a client by accident again!