Introducing: Bounce and Block Classifications

For a variety of reasons, your message may simply not be delivered. It could be as simple as the recipient’s inbox is full, or it could be a little more complicated, such as a reputation issue. Any time this happens it is a missed opportunity of traffic to your website or a purchase on your website. Understanding why your message was blocked or returned is incredibly insightful information available to you.

What are bounce and block classifications?

Fortunately, the mailbox providers will send you an automatic notification of the delivery failure. This message contains the RFC code and the reason for the bounce or lock. A 5XX RFC code represents hard bounces (permanent errors), while a 4XX RFC code represents blocks.

However, not all Internet Service Providers (ISPs) strictly adhere to this code, so there can be exceptions to this rule. As a result, each of these answers helps tell a story about potential problems that you may need to troubleshoot with your email program.

The downside is that there are myriad types of block and bounce reactions. And it is not always easy to interpret cryptic response codes and messages from mailbox providers. For example, Twilio SendGrid receives hundreds of millions of error messages every day listing many reasons for an email not being delivered because mailbox providers have millions of different ways to tell us what happened.

The good news is that Twilio SendGrid is happy to announce bounce and block classifications.

Our bounce and block classifications will help you understand the various Simple Mail Transfer Protocol or SMTP responses we receive from mailbox providers on your behalf. Just navigate to Statistics> Insights into deliverability to watch your bounce and block data.

Twilio SendGrid bounce and block classifications

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

Invalid address

The invalid address classification groups SMTP response codes that indicate that the recipient’s email address is syntactically incorrect or that the recipient’s mailbox does not exist. It is important to note that email addresses that were valid at some point in the past can become invalid at any time. Some mailbox providers periodically clean up mailboxes that have not been used for a long time. This means that an email address may expire today even if you successfully delivered emails to that email address yesterday.

Technical error

The technical error classification groups rejections that were caused by a technical problem. It is important to note that rejections with this classification can indicate a technical problem on the sender side or on the side of the mailbox provider. Unfortunately, in most cases it is difficult or impossible to determine what the problem is. Here are some common causes of technical failure rejection.

  • The sender is missing important Domain Name System or DNS entries
  • The sender fails authentication


The content classification groups rejections, indicating that the mailbox provider believes the content of your messages is potentially spam, malicious, or otherwise untrustworthy. Mailbox providers can reject messages for a variety of content-related reasons. Here are some common ones.

  • Links or urls within the content are suspicious
  • Messages are too long
  • Attachments are too big / not allowed
  • The text-to-image ratio is deactivated
  • Content appears to be spam or malicious


The reputation classification groups rejections, which indicate that the mailbox provider rejects your message based on the reputation of the sending domain or the sending IP address. Mailbox providers build 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 that the mailbox provider cannot or does not want to process the number of messages you want to send because you are sending too many messages. You may also be sending them too quickly.

Mailbox not available

The Mailbox Unavailable classification groups rejections that indicate that the recipient’s e-mail server cannot deliver the e-mail due to a problem with a single recipient’s mailbox. It is important to note that these are rejections not Indicate that the address you were trying to reach was invalid. It simply means that it is currently unavailable and will not accept messages.


The unclassified classification groups indecipherable rejection messages. Most mailbox providers return helpful, well-written rejections that will give you an 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 get such ambiguous rejections, we put them in 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 might reject a message because we have assigned one of these classifications to every single response we receive from them.

If you need help solving a content problem, reputation problem, etc. our team at is here to help Deliverability Advisor would love to work with you to fix these issues so that your emails get back into your recipient’s inbox. For more information on bounce and block classifications, see our Bounce and block documents.

Comments are closed, but trackbacks and pingbacks are open.