Email Bounce Back: Hard Bounce vs Soft Bounce and Why It Matters

Short answer: An email bounce back is a message that could not be delivered and was returned to the sender with an error. A hard bounce is a permanent failure, most often because the address does not exist. A soft bounce is a temporary one, such as a full mailbox or a busy server. Bounces matter beyond the lost message, because a sender with many of them looks careless to mailbox providers and its other email starts to suffer. A provider should verify lists before sending, remove hard bounces at once and report the bounce rate to you.

• What it is: an email the receiving system refused or could not deliver.
• Two kinds: hard bounces are permanent, soft bounces are temporary.
• What to check: how a provider verifies lists and what it does when bounces rise.

What is an email bounce back?

When an email is sent, the receiving mail system either accepts it or refuses it. If it refuses, it sends back a short automated notice saying the message was not delivered and, usually, why. That notice is the bounce. You have probably seen one after mistyping an address: a reply from a “mail delivery” system with an error code and a line such as “address not found”.

In a campaign, the sending tool reads these notices automatically and counts them. The bounce rate is the share of sent emails that came back this way. It is one of the few figures in email that is measured directly, which makes it a dependable sign of how good a list is.

What is the difference between a hard bounce and a soft bounce?

Every bounce falls into one of two groups. The difference is whether trying again could ever work.

Hard bounce

A hard bounce means the email cannot be delivered to that address, now or later. The person may have left the company, the address may have been typed or guessed wrongly, or the domain may no longer exist. The receiving system is saying “there is nobody here”. Sending again will get the same result, so the address should come off the list for good.

Soft bounce

A soft bounce means the address appears valid but the message could not be delivered at that moment. The mailbox may be full, the receiving server may be down or overloaded, the message may be too large, or the receiver may be slowing down mail from a sender it does not yet trust. Sending systems normally retry soft bounces automatically for a period. If an address keeps soft bouncing across several attempts, it should be treated like a hard bounce and removed.

The line between the two is not always clean. When a receiving system blocks a message because it distrusts the sender, the notice may be worded as either kind. That sort of bounce is not about the address at all. It is a warning about the sender’s reputation, and it needs a different response.

Why are your emails bouncing back?

If you are seeing emails bouncing back in a campaign, the cause is nearly always one of these.

• Old data. People change jobs, and their old addresses are closed. A list that was accurate last year will not be accurate now.
• Guessed addresses. Some data sources work out an address from a name and a company pattern without confirming it exists.
• Typing and formatting errors. A missing letter or a wrong domain ending is enough.
• Full or unused mailboxes. The address exists, but nobody is clearing it.
• Temporary server problems. The receiving system was unavailable when the email arrived.
• Blocks. The receiving system refused the message because of the sender’s reputation, missing authentication or the content.

The first three are list problems, and they are the most common. That is why the answer to most bounce trouble lies in how the list was built and checked, not in the email itself. Where the contacts come from matters too, which we cover in our guide to B2B contact data.

Why does bounce rate matter to a sender’s reputation?

Mailbox providers judge senders by their behavior. A sender who repeatedly writes to addresses that do not exist looks like someone who bought or scraped a list and does not know the people on it. That is how spammers behave, so a high bounce rate counts against the sender.

The damage spreads. Once a sending domain’s reputation falls, its email to valid addresses is more likely to be filtered into spam as well. A list with many bad addresses does not only waste those sends. It lowers the inbox placement of every other message from the same sender. Reputation is one of the main factors in cold email deliverability, and bounces are one of the fastest ways to harm it.

Bounces also distort your other numbers. An open or reply rate should be worked out on emails that were actually delivered, not on emails sent. If bounces are high, a rate based on sends will understate how the campaign did with the people it reached. The email open rate calculator shows how removing bounces changes the result.

We do not quote a “safe” bounce rate here, because mailbox providers set their own rules and change them. The useful principle is simple: lower is better, and a sudden rise needs an explanation.

What should a provider do about bounces?

Bounces cannot be removed entirely. Even a freshly checked list will have a few, because addresses stop working every day. What a provider can control is how few there are and how it reacts.

• Verify every address before sending. Verification tools check whether an address is likely to accept mail without sending a real email. Addresses that fail are removed. This step is called email list scrubbing.
• Handle uncertain addresses with care. Some mail systems accept every address at first and bounce later, so verification cannot confirm them. A provider should have a clear policy for these.
• Remove hard bounces immediately. An address that hard bounces should never be emailed again.
• Limit retries on soft bounces. A few attempts, then removal.
• Watch the bounce rate while the campaign runs. A rise should lead to a pause and a check of the list, not more sending.
• Report it. The bounce rate belongs in your regular report, alongside replies.

What should you ask a provider about bounces?

• How do you verify the list, and when? Verification should happen shortly before sending, not months earlier.
• What do you do with addresses that cannot be verified?
• What happens when an address bounces?
• At what point would you pause a campaign? You want to hear that a rule exists.
• Will I see the bounce rate in my report?
• If the list bounces heavily, who pays to fix it?

Bounces feed straight into your email reputation, which is also why volume per inbox is kept low. See how many cold emails to send per day.

Frequently asked questions

Is a bounce the same as landing in spam?

No. A bounced email was refused and came back with an error. An email in spam was accepted and then filed out of sight. The sender is told about a bounce but not about spam filing.

Can a valid address hard bounce?

Occasionally. A receiving system can return a permanent error because it is blocking the sender, not because the address is wrong. If many addresses at one company or mailbox provider bounce at once, a block is the more likely cause.

Should bounced addresses ever be emailed again?

Hard bounces, no. Soft bounces can be retried a limited number of times. After that they should be removed like any other bad address.

Does verifying a list remove every bounce?

No. It removes most of the avoidable ones. Addresses go out of use every day, and some mail systems cannot be checked in advance, so a small number of bounces is normal.

Who is responsible for bounces when I outsource?

Whoever builds and verifies the list. If that is your provider, list quality is part of what you are paying for, and it is fair to ask how it is checked. Next Reply builds the list in every plan, and you approve it before anything sends.

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