What is a Bounce

How does Bounce processing work

Bounce categories

Bounce classification table


This document will explain the various bounce categories and how the classification of bounces works. Since there is no industry standard for bounce classification, every provider determines bounces based on some common ground but ultimately decides what is a soft, hard and block bounce. This is not static and might change based on the bounce data sent back from Internet Service Providers (ISPs), Mailbox Providers (MBPs) and recipient servers that tell Ometria why an email got rejected. 

What is a Bounce?

A bounce is either a returned email or a direct response when sending email during the SMTP/ESMTP conversation with some text and a numerical code which explains why the sender’s email could not be delivered to the recipient. 

How does Bounce processing work?

When bounces are received, they go through what’s typically known as a bounce processing engine which consists of rules which determine what the bounce category is, what the type is and what description it matches. Please see the Bounce Classification table below. 

Bounce Categories

Soft – Emails that have been temporarily rejected and bounce due to various reasons, generally on the recipient server side. These are safe to resend where they might get delivered on the next try and the email address does not get added to the suppression list. An example being the mailbox is full.

Hard – Emails that bounce where the recipient server tells us there is a permanent delivery failure and retrying typically does not work. Hard bounces are added to the suppression list. An example being a typo in the domain (me@gmial.com), the email doesn’t exist, or it no longer exists.

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

Bounce Classification Table

Bounce Classification


Bounce Category

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)

We have included some questions that are frequently asked. 

  1. 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).

  2. I want to investigate my bounce rate, what should I do? Please raise a support ticket by emailing directly to support@ometria.com or contacting your CSM.

  3. What is SMTP/ESMTP? The Internet Protocol used to technically send out emails and communicate with servers that send and receive email. 

  4. What is an Internet Service Provider (ISP) & 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 meaning 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.