All about Bounces

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 an 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 (, 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 Description Bounce Category
Invalid Recipient The recipient is invalid. Hard
Soft Bounce The message soft bounced. Soft
DNS Failure The message bounced due to a DNS failure. Soft
Mailbox Full The message bounced due to the remote mailbox being over quota. Soft
Too Large The message bounced because it was too large for the recipient. Soft
Timeout The message timed out. Soft
Generic Bounce: No RCPT No recipient could be determined for the message. Hard
Generic Bounce The message failed for unspecified reasons. Soft
Mail Block The message was blocked by the receiver. Block
Spam Block The message was blocked by the receiver as coming from a known spam source. Block
Spam Content The message was blocked by the receiver as spam. Block
Prohibited Attachment The message was blocked by the receiver because it contained an attachment. Block
Relaying Denied The message was blocked by the receiver because relaying is not allowed. Block
Auto-Reply The message is an auto-reply/vacation mail. Soft
Transient Failure Message transmission has been temporarily delayed. Soft
Unsubscribe The message is an unsubscribe request. Hard
Challenge-Response The message is a challenge-response probe. Soft

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 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 MBP provides email mailboxes for “free”. Free being, you provide your personal details to then be used for advertising as there is no charge for having a mailbox unless you upgrade for more storage for example.