The Spamhaus Project


How to handle bounced emails

Discover how to manage hard bounces, soft bounces and ISP hard blocks. All components of bounced email management help email deliverability.

by The Spamhaus TeamFebruary 15, 20226 minutes reading time

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Managing hard bounces, soft bounces, and blocks is another crucial element of maintaining a successful email marketing program. Here’s an explanation of the difference between hard and soft bounces, ISP hard blocks, and what you should be doing when you encounter them during an email campaign.

Hard Bounces | Definition of a hard bounce – Fatal Error: No retry will occur

A hard bounce occurs when the email server rejects the email due to permanent conditions. This typically results when ‘user unknown’ or ‘domain not found’ errors occur. However, there are also other, less common reasons.

The SMTP response code for all hard bounces begins with 5xx. Below are some of the most frequently seen by marketers:

  • 550 – “Non-existent email address”****: This usually defines a non-existent email address on the remote side. The great majority of errors 550 mean that the recipient’s email address doesn’t exist.
  • 512A DNS error: the host server for the recipient’s domain name cannot be found. The domain does not exist in DNS.
  • 551 – “User not local or invalid address – Relay denied.” Meaning if both your address and the recipient’s are not locally hosted by the server, a relay can be interrupted.
  • 552 – “Requested mail actions aborted – Exceeded storage allocation”: simply put, the recipient’s mailbox is full.
  • 553“Requested action not taken – Mailbox name invalid.” There is an invalid email address in the “recipient” field.

In relation to an email marketing campaign, most of these errors are the direct result of poor data hygiene. As there is a considerable amount of churn in email addresses, sending to outdated lists increases the chances of sending to non-existent domains. This can be dangerous as non-existent domains can be spamtraps. If you encounter a hard bounce, ensure you don’t send to the contact again.

Soft Bounces | Definition of a soft bounce: Temporary Failure: retry will occur

A soft bounce occurs when the email server rejects the email due to a normally temporary condition, such as a full inbox or too many complaints. When that happens, the system tries to send the email again until it is either accepted or times out. The time-out is set on the sending side and varies by network.

  • 421 – The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, a lack of resources on the receiver side, or a more general temporary problem such as a reduction in reputation.
  • 450 – “Requested action not taken – The user’s mailbox is unavailable.” The mailbox has been corrupted or placed on an offline server, or the email hasn’t been accepted due to reputation or blocklisting.
  • 451 –Requested action aborted – Local error in processing.” The ISP’s server or the server that got the first relay has encountered a connection problem.
  • 452 – Too many emails sent or too many recipients: more in general, a server storage limit exceeded.

Some ISPs respond to dips in reputation by issuing “tempfails” preceded by a 4xx code, usually 421. This means they will defer mail for a determined amount of time until either the mail stops coming or reputation re-calculates upward again. That error can look like this: “421 4.7.0 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints.”

Addresses affected by a temp-fail should not necessarily be removed from a mailing list! If the block is due to a poor-quality mailing list, you should review your list hygiene and correct any problems before trying again.

ISP Hard Blocks

In the modern email ecosystem, most receivers base their spam filtering on a reputation-based system, be it purchased, home-grown, or a combination of the two. They all have different ways of accomplishing their goal of protecting their users from unwanted email, but they all have some commonalities. ISPs typically respond to mail that is coming from an IP/domain with poor reputation and/or incorrect or missing authentication in this general pattern:

  • Placing mail in the spam folder.
  • Increasing levels of rejection, beginning with temp-failing or throttling the problematic mail stream.
  • Bouncing/rejecting the problematic mail stream and possibly escalating to hard blocking.

If an ISP issues a hard block, they are indicating they will no longer accept mail from the sending IP and that this is not a temporary issue. The most commonly used SMTP code for hard blocks is:

  • 554 – This is a permanent error, and the server will not try to send the message again.

This type of bounce occurs when the receiving email server rejects the email for policy reasons, including:

  • URL or email body content blocks
  • Lack of correct authentication
  • Poor IP or domain reputation
  • The presence of the sending IP/domain on a blocklist in use by the recipient.

Depending on the ISP, the blocks will often disappear over an arbitrary period (24-72 hours, generally) if the triggering issue ceases.

The error for such a block will usually begin with “554” and contain more specific information or a URL you can consult for more details. For example: “554 The IP address of your mail server ( was found in the Spamhaus blocklist. See for details.

These blocks can result from a decision made by looking at their own data, data provided by a reputation provider like Spamhaus, a filter vendor such as Cloudmark, or a combination of two or more of these.

If the block does not resolve on its own, but you have resolved the triggering issue, you will need to open a ticket with the network that is issuing the block to fix the problem, if possible. It will be necessary to understand what caused the block and correct it before opening a ticket. Some ISPS, notably Gmail, have no remediation path available at all.

Correctly managing bounces goes a long way toward keeping mailing lists clean and up to date.

Spam filters and Reputation Providers

As a general rule, it is safe to say that Gmail, Hotmail, and Verizon Media (what used to be Yahoo and AOL) use mostly home-grown spam filtering technology. Due to the nature of their business model as mailbox providers, they have access to a huge amount of data regarding their own users’ email behavior. Generally, these providers prefer to use their data, as it is more accurate in relation to their networks. They may or may not use commercially available data combined with their own: they will not say.

Providers like Comcast, RoadRunner, and Sky use commercial spam filters and reputation providers such as Cloudmark’s and Spamhaus’ offerings.

Modern spam filters have become very sophisticated; they are flexible and fast. Being blocked by any spam filter vendor (such as Cloudmark) or reputation provider (such as Spamhaus) can have time-consuming and costly results.

Of course, the impact depends on many factors, including how widespread the adoption of the filter is. There are hundreds of blocklists in the industry, but only a few have a broad impact. Even if you use a comprehensive blocklist checking tool to review your IP or domain (MXToolbox, for example), you will end up on a blocklist somewhere. How seriously to take that depends on who is issuing the block. For example, a listing on the Spamhaus SBL has enormous reach, whereas you can happily ignore a listing by SPEWS.

Now you know how to handle bounced emails let’s review how to manage complaints.