Why Do Emails Bounce?



A bounced email is an undelivered email that you sent to a recipient, but was returned to you by the recipient’s email server. The email is returned with a message, called an 'SMTP reply', which should give a reason for why the email bounced. Your email server will scan this reply and try to classify the bounce based on the reason supplied. Unfortunately, there's no standard format for these replies and sometimes the server will classify the bounce incorrectly.

What the SMTP Reply Looks Like

Below is an example of an SMTP reply received from the Gmail server. It contains an error code and a description of why the email wasn't delivered successfully.

Sometimes these SMTP replies aren't readable by the system and you'll need to read through them to determine why the email bounced. This happens because of the lack of industry standards.


There are two types of bounces:

  • Soft bounce
  • Hard bounce

Soft Bounce

A soft bounce is a temporary error which occurs when an email is accepted by the recipient's mail server, but isn't delivered to that specific mail box. Once the mail server receives the message, it checks that the email address actually exists on the system, and whether that mailbox is allowed to receive mails.

Reasons For a Soft Bounce

If your email is bounced, you'll receive an email notification with a reason for the bounce. Soft bounces can occur for the following reasons:

  • The recipient's mailbox is full. If your recipient has too many emails in their inbox, your email will not be delivered.
  • The receiving email domain may be temporarily unavailable.
  • Your message is too large or you may have attached images that exceed the recipient's mail server limits. Some servers have email size policies and mails will not be allowed through if they exceed this size.

Hard Bounce

In the case of a hard bounce, non-delivery is due to a permanent error. This email is returned without being accepted by the recipient's mail server.

Reasons For a Hard Bounce

Hard bounces can occur for the following reasons:

  • The recipient’s address does not exist. For example, if you tried to send to 'support@example.com' and received this error, it indicates that there is no mailbox called 'support' on the 'example.com' domain.
  • The recipient’s domain does not exist. This indicates that the domain, 'example.com' in our example above, does not exist.

Bounce Handling

Most email marketing programs have pre-determined methods for automatically dealing with bounced emails. These systems handle soft and hard bounces differently.

The system may try to re-send an email which has been soft bounced. Usually there's a bounce limit set, based on number of bounces or a period of time during which the mail consistently fails.

Most email marketing systems will automatically suspend an email address that returns a hard bounce. This is to avoid developing a bad reputation with the ISPs and possibly being marked as a source of spam.

What You Can Do To Reduce Bounces

There are two important ways in which you can reduce the number of bounces your email campaign receives:

  1. Make sure you're only sending emails to people who want to receive communications from you. You can do this by sending confirmation emails or requiring a double-opt-in for contacts on your database.
  2. Perform regular database management to make sure that you're only sending emails to addresses that exist.

Bounce Reasons

This table highlights some of the most common reasons why your email may be bounced.

Bounce Reason Description
Bad configuration Messages rejected due to configuration issues with remote host.
Bad connection Messages bounced due to bad connection issues with remote host.
Bad domain Messages bounced due to invalid or non-existent domains.
Bad mailbox Messages rejected due to invalid or non-existent recipient addresses.
Content-related Messages refused or blocked due to content-related reasons.
Inactive mailbox Messages rejected due to expired, inactive, or disabled recipient addresses.
Invalid sender Messages bounced due to invalid DNS or MX entry for sending domain.
Message expired Messages bounced due to not being delivered before the bounce-after time.
No answer from host Messages bounced due to receiving no response from remote host after connecting.
Other Messages rejected due to other reasons.
Policy related Messages refused or blocked due to general policy reasons.
Protocol errors Messages rejected due to SMTP protocol syntax or sequence errors.
Quota issues Messages rejected or blocked due to mailbox quota issues.
Relaying issues Messages refused or blocked due to remote mail server relaying issues.
Routing errors Messages bounced due to mail routing issues for recipient domain.
Spam-related Messages refused or blocked due to spam-related reasons.
Virus-related Messages refused or blocked due to virus-related reasons.


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