How does Bounce processing work

Bounce categories

Bounce classification table


A bounce is either a returned email or a direct response when sending email which explains why the sender’s email could not be delivered to the recipient. 

This page explains how Ometria defines bounces and bounce categories. 

These categories are not static and may change based on the bounce data sent back from Internet Service Providers (ISPs), Mailbox Providers (MBPs) and recipient servers which tells Ometria why an email bounced. 

How does bounce processing work?

When bounces are received, they go through a bounce processing engine which consists of rules which determine what the bounce category is, what the type is and which description it matches. 

Bounce categories

Soft – Emails that have been temporarily rejected and bounce for various reasons, generally on the recipient server side. 

A common reason for emails soft bouncing is the recipient's mailbox being full. Soft bounces are safe to resend and may get delivered on the next try. Soft bounces are temporary in nature, so they are not added to the suppression list. 

Hard – Emails that bounce and the recipient server tells us there is a permanent delivery failure and retrying typically does not work. Common reasons for hard bounces include:

  • a typo in the email address (e.g.,
  • a deactivated email address (i.e. it no longer exists).

Hard bounces are permanent in nature and suggest that the email address will never be successfully delivered to, so these emails are added to the suppression list and emails will not be sent to those addresses again. 

Blocked – Emails that were refused because it was determined that the content is spammy, the IP has a poor reputation or a number of other filter verdicts. These emails are rejected at the server level and never reach the intended recipient. 

A common reason for a blocked bounce is the sender being on a blacklist or has low reputation scores with the destination inbox provider. Ometria does not add blocked bounces to the suppression list as they may be temporary in nature.

Bounce Classification Table

Bounce typeDescriptionCategory

Invalid Recipient

The recipient is invalid.


Soft Bounce

The message soft bounced.


DNS Failure

The message bounced due to a DNS failure.


Mailbox Full

The message bounced due to the remote mailbox being over quota.


Too Large

The message bounced because it was too large for the recipient.



The message timed out.


Generic Bounce: No RCPT

No recipient could be determined for the message.


Generic Bounce

The message failed for unspecified reasons.


Mail Block

The message was blocked by the receiver.


Spam Block

The message was blocked by the receiver as coming from a known spam source.


Spam Content

The message was blocked by the receiver as spam.


Prohibited Attachment

The message was blocked by the receiver because it contained an attachment.


Relaying Denied

The message was blocked by the receiver because relaying is not allowed.



The message is an auto-reply/vacation mail.


Transient Failure

Message transmission has been temporarily delayed.



The message is an unsubscribe request.



The message is a challenge-response probe.


Frequently Asked Questions (F.A.Q)

Why do you currently only show the soft and hard bounce rate? 

We are currently investigating the inclusion of the block bounce rate and do not have an exact date of when this will be implemented. we advise you send a feedback request to your customer success manager (CSM).

I want to investigate my bounce rate, what should I do?

Please raise a support ticket by emailing directly to or contacting your CSM.

What is SMTP/ESMTP? 

The Internet Protocol (IP) used to send out emails and communicate with servers that send and receive email. 

What is an Internet Service Provider (ISP) and Mailbox Provider (MBP)? 

An ISP is a provider of a mailbox as part of a paid service, whereas an MBP provides email mailboxes for 'free'. 

'Free' means that you provide your personal details to them to be used for advertising, and there is no charge for having a mailbox unless you upgrade.