“An email bounce is the technological equivalent to getting a letter marked ‘return to sender’.” – Litmus

We don’t need to tell you that deliverability is crucial in any email marketing campaign. You know that – and you certainly don’t want your emails to bounce.

What you might be less sure about is precisely why they are bouncing and what that means. To help you figure it out, let’s take a look at two important terms – hard vs soft bounces.

In this article, we’ll answer the following questions about hard vs soft bounces:

  • What do we mean by hard vs soft bounces?
  • Why it’s important to know the difference between hard vs soft bounces?
  • What to do with your bounce data?

Let’s begin, shall we? 

What is a Hard Bounce?

Emails “bounce” when they can’t be delivered. If it’s a hard bounce, that means they permanently can’t be delivered.

There are a whole bunch of reasons behind this. For example, it could have been a work email address that no longer exists because the person has moved on. It could have been a fake email address. Or, it could simply be that there’s a spelling mistake somewhere in it.

The point is, that email is never going to work, no matter how many times you try it.

What to Do About Hard Bounces?

Since there’s nothing you can do to make this email address work, you might as well take it off your list. Scratch that – you need to take it off your list before it starts impacting on your trust ratings or skewing your campaign performance data.

What Is a Soft Bounce?

An email that “soft” bounces also can’t be delivered – but it’s only a temporary issue.

The email address exists, so the problem is elsewhere. Either it’s an issue with the inbox, which may be too full or having technical problems. Or there’s an issue with the email itself – it’s too large, for example.

What to Do About Soft Bounces?

The good news is that many email providers don’t give up at the first hurdle. They’ll keep trying to deliver your email over the next couple of days. You don’t necessarily need to do anything.

That said, it’s well worth keeping a close eye on these email addresses. If they consistently soft bounce, it may be worth removing them anyway. The email account may not be in constant use, in which case your email is unlikely to be opened anyway. What’s the point in emailing someone who won’t see, open or derive any value from your email anyway?

Final Thoughts

Tracking your bounce rates isn’t just about deciding whether or not to remove an email from your list. It also gives you important clues as to whether you’re making self-defeating mistakes during your sign-up and list-building activities.

Psst! Get top tips for building your email list here >

For example, if you’re getting a lot of hard bounces, take a good hard look at these to figure out whether they’ve simply expired, whether they’re fake, or whether there might be a typo in there somewhere.

For example, fake emails show that people don’t want to receive marketing emails from you. Either you didn’t convince them that this would be valuable to them or you got their email addresses through sneaky means (like buying them)… which you should never do!

If you have tons of emails with mistakes in, what’s going wrong in your sign-up or data entry process? Is the form fiddly, difficult to use or displaying weirdly? Are you taking email addresses by hand and manually entering them into the system later? A few simple tweaks could make a big difference!

Remember that, ideally, you should keep your overall bounce rate under 2%. Pruning lists, analyzing why email addresses fail and second-guessing what’s underpinning a high bounce rate will take up far more of your time than adopting sensible approaches in the first place. It all comes down to one thing: making sure that everyone on your list wants to be there in the first place!

We advise to segment your list every six months by those that have opened your emails to ensure good reputation and engagement.

4XX Soft Bounce and Hard Bounce Code Lookup and Definitions

CodeTypeDescription
421SoftService not available
450SoftMailbox unavailable
451SoftError in processing
452SoftInsufficient system storage

5XX Soft Bounce and Hard Bounce Code Lookup and Definitions

CodeTypeDescription
500HardAddress does not exist
510HardOther address status
511HardBad destination mailbox address
512HardBad destination system address
513HardBad destination mailbox address syntax
514HardDestination mailbox address ambiguous
515HardDestination mailbox address valid
516HardMailbox has moved
517HardBad sender’s mailbox address syntax
518HardBad sender’s system address
520SoftOther or undefined mailbox status
521SoftMailbox disabled, not accepting messages
522SoftMailbox full
523HardMessage length exceeds administrative limit
524HardMailing list expansion problem
530HardOther or undefined mail system status
531SoftMail system full
532HardSystem not accepting network messages
533HardSystem not capable of selected features
534HardMessage too big for system
540HardOther or undefined network or routing status
541HardNo answer from host
542HardBad connection
543HardRouting server failure
544HardUnable to route
545SoftNetwork congestion
546HardRouting loop detected
547HardDelivery time expired
550HardOther or undefined protocol status
551HardInvalid command
552HardSyntax error
553SoftToo many recipients
554HardInvalid command arguments
555HardWrong protocol version
560HardOther or undefined media error
561HardMedia not supported
562HardConversion required and prohibited
563HardConversion required but not supported
564HardConversion with loss performed
565HardConversion failed
570HardOther or undefined security status
571HardDelivery not authorized, message refused
572HardMailing list expansion prohibited
573HardSecurity conversion required but not possible
574HardSecurity features not supported
575HardCryptographic failure
576HardCryptographic algorithm not supported
577HardMessage integrity failure

MISC Bounce Code Lookup and Definitions

CodeTypeDescription
911HardHard bounce with no bounce code found It could be an invalid email or rejected email from your mail server (such as from a sending limit)

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