Introducing: Bounce and Block Classifications

October 06, 2021
Written by
Denis O'Sullivan
Opinions expressed by Twilio contributors are their own

Introducing: Bounce and Block Classifications

For several reasons, there are times when your message simply doesn’t get delivered. It could be something as simple as the recipient’s inbox is full, or it could be something more complicated—such as a reputational issue. Every time that happens, it’s a missed opportunity for traffic to your site or a purchase on your site. Understanding why your message was blocked or bounced is incredibly insightful information to have at your disposal. 

What are bounce and block classifications?

Thankfully, mailbox providers give you an automatic notification of the delivery failure. This message contains the RFC code and the reason for the bounce or block. A 5XX RFC code depicts hard bounces (permanent failures), while a 4XX RFC code depicts blocks. 


However, not all internet service providers (ISPs) adhere to that code consistently, so there could be exceptions to this rule. What is consistent is that each of these responses helps tell a story about potential issues you might need to fix with your email program. 

The downside is that there are countless types of block and bounce responses. And it’s not always easy trying to interpret cryptic response codes and messages from mailbox providers. For example, Twilio SendGrid receives hundreds of millions of failure messages every day detailing many reasons an email might fail to deliver, as mailbox providers have millions of different ways to tell us what happened. 
The good news is Twilio SendGrid is happy to announce bounce and block classifications.
Our bounce and block classifications will help you make sense of the different Simple Mail Transfer Protocol or SMTP responses that we receive from mailbox providers on your behalf. Simply navigate to Stats > Deliverability Insights to observe your bounce and block data. 

Twilio SendGrid’s bounce and block classifications

Bounce and block classifications save you from having to sift through thousands or even millions of rejection messages by conveniently bucketing all of these responses into seven simple classifications. Read on to learn about each classification type.

Invalid address

The invalid address classification groups SMTP response codes that indicate the recipient’s email address is syntactically incorrect or the recipient’s mailbox doesn’t exist. It’s important to note that email addresses that were valid at some time in the past can become invalid at any time. Some mailbox providers will do periodic purges of mailboxes that haven’t been used in a long time. This means that an email address could become invalid today even if you successfully delivered mail to that email address yesterday.

Technical failure

The technical failure classification groups rejections caused by a technical problem. It’s important to note that rejections with this classification can indicate a technical problem on the sender side or the mailbox provider’s side. Unfortunately, in most cases, it’s difficult or impossible to determine where the problem lies. Here are some common causes for technical failure rejections.
  • The sender is missing important domain name system or DNS records
  • The sender is failing authentication


The content classification groups rejections that indicate the mailbox provider believes the content of your messages is potentially spammy, malicious, or otherwise untrustworthy. Mailbox providers can reject messages for a number of content-related reasons. Here are some common ones.
  • Links or URLs within the content are suspicious
  • Messages are too lengthy
  • Attachments are too large/not allowed
  • Text to image ratio is off
  • Content appears spammy or malicious


The reputation classification groups rejections that indicate the mailbox provider rejects your message due to the reputation of the sending domain or sending IP address. Mailbox providers establish your sender reputation using complex algorithms that use hundreds or even thousands of signals to determine your trustworthiness as a sender.

Frequency or volume too high

The frequency or volume too high classification groups rejections that indicate the mailbox provider is unable or unwilling to process the number of messages you’re trying to send because you’re sending too many messages. It could also be that you’re sending them too quickly.

Mailbox unavailable

The mailbox unavailable classification groups rejections that indicate the recipient’s mail server is unable to deliver the email due to a problem with an individual recipient’s mailbox. It’s important to note that these rejections don’t indicate that the address you tried to reach was invalid. It simply means it’s currently unavailable and not accepting messages.


The unclassified classification groups indecipherable rejection messages. Most mailbox providers return helpful, well-written rejections that give you some idea of what the problem is, but some mailbox providers don’t. For example, rejections like “550 there was a problem” are common. When we receive rejections that are this ambiguous, we place them into the unclassified bucket.

Land in inboxes by understanding classifications

Gone are the days of trying to interpret cryptic response codes and messages from mailbox providers. These 7 classifications capture every possible reason a mailbox provider can reject a message, as we’ve mapped each unique response we get from them to one of these classifications. 


If you would like assistance figuring out a content issue, reputation issue, etc., our team of Deliverability Consultants would be happy to work with you to remedy those issues so that your emails start landing back in your recipient’s inbox. For more information on bounce and block classifications, be sure to check out our bounce and block docs

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