Understanding Hard and Soft Bounces

What a bounce is

A bounce happens when an email cannot be delivered and the receiving system sends a failure notice back. It is different from a message that arrives but lands in spam; a bounce means the message was refused or could not be delivered at all. Your email service records these notices and shows them in campaign reports.

Bounces matter because mailbox providers see how often you try to send to addresses that do not work. Frequent failures suggest careless list management, and this can weigh on your sender reputation. Handling bounces promptly is a basic part of protecting your deliverability.

Hard bounces

A hard bounce is a permanent failure. Typical causes include:

• The address does not exist, for example because of a typo or because the mailbox was closed.
• The domain does not exist or cannot receive mail.
• The recipient’s server has permanently refused your mail.

How to handle them: Stop sending to the address straight away. Most email services suppress hard-bounced addresses automatically, but confirm yours does. Never re-import them from an old spreadsheet, since doing so will cause the same failure again. If the bounce came from an obvious typo, such as a misspelt domain, you might correct it only if you can verify the person’s intent, for example if they are a known customer you can contact another way. Otherwise leave the address out.

Soft bounces

A soft bounce is a temporary failure. Common causes include:

• The recipient’s mailbox is full.
• The receiving server is down or busy.
• The message is too large.
• A temporary block or limit, which may be related to your sending volume or reputation.

How to handle them: Email services generally retry soft bounces for a period before giving up. One soft bounce is usually no cause for concern. If the same address soft bounces across several campaigns in a row, it is probably abandoned, and you can treat it as a hard bounce. How many repeats you tolerate is a judgement call; a common approach is to review addresses after a handful of consecutive failures, but check your email service’s default and adjust to your sending frequency.

If many soft bounces appear at once, especially all at one mailbox provider, look at your sending rather than the addresses. It could mean the provider is limiting your mail, perhaps because volume rose too fast or because of reputation issues. Slowing down and reviewing authentication, content and list quality is a sensible response.

Reading the codes and reasons

Bounce notices often include a short code and message. In general, codes beginning with 5 point to permanent failure and those beginning with 4 to temporary failure, but providers word their messages differently, and your email service normally translates them into categories for you. You rarely need to interpret raw codes. When you do, the text usually hints at the cause: unknown user, mailbox full, message blocked, and so on.

Some services use extra categories, such as “block” or “technical” bounces, for cases where the problem is with your message or reputation rather than the address itself. Do not remove perfectly good addresses because of a block; investigate the cause instead.

A simple bounce-rate example

Bounce rate is the number of bounced emails divided by the number sent. If you send 10,000 emails and 150 bounce, the rate is 150 divided by 10,000, which is 1.5 percent. The same 150 bounces on a list of 2,000 would be 7.5 percent, which would be a much stronger warning sign. What counts as acceptable varies by service and provider, so consult your email service’s current guidance instead of relying on a single figure. The broader set of email metrics will help you put this number in context.

Where bounces come from

Knowing the likely sources helps you reduce them:

• Typos at signup. Confirmation steps such as double opt-in catch most of them, and well-designed forms help too.
• Old lists. Addresses decay as people change jobs and abandon inboxes, which is why a regular routine of list hygiene matters.
• Purchased or scraped lists. These tend to have high failure rates and raise consent problems. Avoid them.
• Imported data from other systems. Look at your CRM data before importing, and send a small test first if the source is unclear.

A routine for handling bounces

1. After each campaign, check the number of hard and soft bounces.
2. Confirm hard bounces are suppressed.
3. Review addresses with repeated soft bounces, and suppress them.
4. Look for patterns: one provider, one signup source, one date.
5. If the rate is higher than usual, pause and investigate before the next send.
6. Record what you found, so the same problem is recognised next time.

When a new sender sees bounces

A new domain or address that starts sending a large volume will often see more soft bounces and blocks, because providers are cautious with unknown senders. A gradual domain warm-up helps by limiting volume at first and starting with engaged contacts.

Remember that results vary and providers change their systems. If you are unsure what a message means, the support documentation for your email service is the best place to check.