When an email cannot be delivered, it will bounce back to the sender. The email server will return the bounce with reason, typically bounces categorize into two types: Soft bounces and Hard bounces.
In this article, you'll learn the definition of bounces.
Hard bounces
Hard bounce indicates an email cannot be delivered due to some permanent errors that are classified under SMTP 5xx response status codes. We will move these bounced email addresses to Inactive list and they will automatically be excluded from any future campaigns. Below are the reasons hard bounces happen:
| Category | Explanation | Response code |
| Bad domain | The domain of the email address is not valid. For example, instead of hotmail.com they written as hotail.com. | bad-domain |
| Bad mailbox | The email has been rejected because the email address is not valid or the user doesn't exist. | bad-mailbox |
| Inactive mailbox | Typically indicates that the account is disabled or discontinued. | inactive-mailbox |
Soft bounces
Soft bounce indicates temporary delivery issue due to persistent transient failures denoted by SMTP 4xx response status codes. We will continue to retry and attempt delivery and may repeatedly do so for a day or two (depending on our dynamic configurations) before reporting that the mail cannot be delivered. Unlike hard bounces, soft bounced email addresses will not be automatically moved to the Inactive list.
| Category | Explanation | Response code |
| Quota issues | The recipient's email server exceeded the email limits. | quota-issues |
| No answer from host | System could not receive any response from recipient's server. | no-answer-from-host |
| Relaying issues | Email being rejected due to no authorization allow for delivery from recipient's server, usually affects corporate domains. | relaying-issues |
| Policy related | Recipient's mail server rejects the email due to some security policies in place on the recipient's mail server. | policy-related |
| Routing errors | Email with an existing domain but mail service is undefined. | routing-errors |
| Bad connection | Recipient's mail server unable to accept the email due to connection issue with the remote server. | bad-connection |
| Invalid sender | It happens when format of the sender's email address is not correct. | invalid-sender |
| Spam related | Recipient's mail server considers either the email content or the sender is a spammer. This could be due to corporate policies in place on recipient's mail server, or some bad choice of words in your email that being flagged by the their spam filters. | spam-related |
| Virus related | Email blocked due to reasons related to infected emails. Usually happens when you attach a file containing virus or which could potentially contain a threat. For example when you attach Word or Excel file, your email will likely be blocked. | virus-related |
| Content related | Email blocked due to the email content. | content-related |
| Bad configuration | Email rejected due to the configuration of the recipient's server. It could be some errors in the settings and is not accepting new emails. | bad-configuration |
| Protocol errors | Email rejected due to syntax error in SMTP protocol. | protocol-errors |
| Message expired | Email has failed to deliver after maximum time attempts allowed. | message-expired |
| Other | Email rejected due to other reasons. | other |