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What are bounces and spam complaints?

A bounce is when an email is returned back to Postmark’s email servers after being sent.

This can happen for many reasons:

  • Hard Bounce: The email address no longer exists.
  • Soft Bounce: The mailbox is full and not accepting emails.
  • Autoresponder: Automatic email responder including nondescript NDRs and some "out of office" replies.
  • Transient/Undeliverable, Message Delayed, DNS Error: There were problems connecting to the person’s mail server.
  • Challenge Verification: A message was returned asking for approval.
  • Unsubscribe, Address Change: A change of address or unsubscribe was requested.

For a full list of possible bounce types, head here.

How Postmark handles bounces

In Postmark, detailed information for each bounced email is provided. When emails are identified as Hard Bounce or Spam Complaint they are automatically deactivated from your list. The next time you send, these emails will be disregarded. This is done to remain polite to the ISPs and improve delivery, as well as keep your database clean.

Keep in mind that each server’s bounce reports are independent and not associated. Also, deactivated emails help you reduce credit usage since they are no longer sent messages.

To view the detailed bounce information for each email, go to the Activity page for the Stream. At the top on the right-hand side you can filter by any bounce type.

Viewing detailed bounce information

When you open a particular email to check the bounce, click on Hard Bounce (or whatever other type of bounce) to open the full SMTP response. Postmark will categorize bounces solely based on the error response receive from the remote server.

What you can do about bounced emails

Understanding which emails bounce will help you react and improve customer response. For instance, if your invoice emails are bouncing to a customer, you can notify them in your site to update their profile. Bounces also help you understand if emails are getting blocked for content reasons. Here are some specific details…

False positives

In some cases, an email might hard bounce, but still be valid due to the way the server rejected the message. If you feel an email was bounced incorrectly, you can activate the email again by clicking on the Reactivate button next to the email address. We also provide a notify option to help us resolve any issues with our bounce parser. We appreciate your feedback.

Address Change

If an address change is requested, it is best to contact those subscribers individually and ask for the new email address. You can then edit the subscribers information in your own database to make sure your list is up to date.

Challenge Verification

Some subscribers sign up for services that force you to confirm your identity (email) before they accept emails from you. This is called a Challenge/Response. If you noticed some bounced emails with this status, we also recommend that you contact these subscribers individually to be granted sending access to the person’s email.

Spam Complaints

A spam complaint is recorded when a subscriber click This is Spam or Mark as Spam in email clients like Yahoo, Hotmail, AOL, and other similar services.

While you should not have any spam complaints, they do happen. Once a spam complaint is recorded, Postmark will deactivate this address and will not let you reactivate it. In the email industry, spam complaints are a clear metric to determine abuse and poor sending practices. It’s important that we take these reports seriously to ensure the best delivery for all customers.

If the recipient reaches out to you or your sender and wants to receive emails again, contact us at and let us know to reactivate the recipient’s address.

Spam Notification

This means that your content was rejected by a mail server. This could be due to rigid corporate policies in place on the receiving mail server or some bad choice of words in your email that are getting flagged by spam filters. (we recommend not mentioning “viagra”) :)

Spam Notifications need to be resolved on the receiving mail server side, specifically with the spam filter settings for the domain you are sending to. To resolve the blocks, contact the recipient through another channel and ask them to ask their IT team or mail administration team to whitelist either your sending domain or Postmark’s sending IP addresses in their spam filter settings.

ISP Blocks

An ISP Block bounce means that the message was rejected by the receiving mail provider and not delivered to the recipient. Most commonly, an ISP blocks happens due to a local policy by the receiving mail server or in rare cases with Postmark, a reputation issue with the IP we used to send the message. More on how ISP Blocks occur and how to resolve them here


The Rebound JavaScript snippet (once installed on your website) will tap into the Postmark API to check for hard bounces and prompt your customers to update their email address if they’ve experienced deliverability issues in the past.

Screenshot of rebound JavaScript snippet

Last updated August 3rd, 2023

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