Why do email bounce notifications differ and why might bounced emails be marked as read?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Jul 2025
Updated 19 Aug 2025
6 min read
It can be perplexing to receive an email bounce notification that doesn't quite match what you're used to. Sometimes a bounce arrives almost instantly, like a message rejected at the doorstep, while other times, it shows up after a delay, perhaps 10 or 15 minutes later, looking entirely different. This variation in bounce notifications often sparks questions about what's happening behind the scenes in email delivery.
Understanding these differences is key to effective email management. It reveals the complex interplay between sending and receiving mail servers, each with its own configurations and rules. The reason for these discrepancies often lies in how mail servers are set up and how they communicate delivery failures, which can range from immediate rejections to deferred responses.
Understanding email bounce types and timing
Email bounces are broadly categorized into two types: hard bounces and soft bounces. A hard bounce indicates a permanent failure, meaning the email cannot be delivered due to reasons like an invalid email address, a non-existent domain, or the recipient's server completely blocking your messages. These usually result in immediate bounce notifications, as the receiving server instantly recognizes that delivery is impossible.
Soft bounces, on the other hand, signify temporary delivery issues. Common causes include a full mailbox, a temporarily unavailable server, or the message size exceeding the recipient's limits. For soft bounces, the sending server might attempt delivery multiple times over a period before finally giving up and sending a bounce notification. This explains why some bounces arrive with a significant delay, as the sending server waits for a potential resolution of the temporary issue.
The timing of these notifications is heavily dependent on whether the bounce is synchronous or asynchronous. Synchronous bounces happen in real-time during the initial SMTP conversation. The receiving server immediately rejects the email, and the sending server gets an instant notification. Asynchronous bounces occur after the receiving server has initially accepted the email for delivery but then fails to deliver it internally. The bounce notification is then generated and sent back to the sender at a later time, leading to a delay.
Synchronous bounces
Immediate feedback: Occur during the initial SMTP transaction.
Server rejection: The receiving server rejects the email outright.
Common causes: Invalid recipient address, domain not found, or immediate blocklist (blacklist) rejection.
Examples: 550 codes indicating User unknown or Relay denied.
Asynchronous bounces
Delayed feedback: Arrive minutes or hours later.
Internal rejection: Mail is initially accepted, then rejected internally by the recipient's server.
Common causes: Mailbox full, server temporary issues, content filtering (spam filters), or a more complex blocklist (blacklist) check.
Examples: Out-of-band bounces, or Deferred messages that eventually fail. You can read more about Out-of-Band (OOB) email bounces.
Why bounce notifications differ
The appearance and content of bounce notifications can vary significantly because there isn't one universal standard for how mail servers generate them. Different Mailbox Providers (MBPs) and email systems implement their own versions of Non-Delivery Reports (NDRs) or Delivery Status Notifications (DSNs).
Some systems might provide detailed SMTP error codes like 550 5.1.1 (mailbox not found), while others offer more general, human-readable messages. The level of detail, formatting, and even the language can differ widely. This is particularly true if the two bounce messages you received came from different recipient domains, as each domain's mail server configuration dictates its bounce behavior.
Common bounce message elements
Sender information: The email address that generated the bounce.
Recipient information: The undeliverable email address.
Status code: Numerical codes like 5.1.1, 5.2.2, or 4.2.2 that indicate the specific problem. You can learn more about SMTP bounce codes and their meanings.
Diagnostic code: Additional details from the remote server about the failure.
Original message headers: Sometimes included for context.
For example, a mail server might respond with a simple User unknown for a hard bounce, while another might provide an extensive report detailing the attempted connections, timeouts, and specific rejection reasons. This is why having tools that can parse and categorize different bounce messages is incredibly valuable for managing your email deliverability effectively. For deeper insight, you can also look into Gmail's documentation on fixing bounced emails.
The curious case of bounced emails marked as read
The most intriguing part of this puzzle is when a bounced email is subsequently marked as read. This seemingly contradictory behavior can occur due to a few scenarios, most commonly involving how mail servers process incoming mail and how email tracking works.
Many email tracking systems rely on a tiny, invisible image pixel embedded in the email. When the email is opened, this pixel is loaded from a server, registering an open event. However, some mail servers and security systems automatically open emails and scan them for malicious content before delivering them to the recipient's inbox. This pre-scanning process can trigger the tracking pixel, recording an open, even if the email ultimately bounces due to a later policy decision or a full mailbox.
Another possibility is a delayed soft bounce where the email was initially delivered to the recipient's server but then encountered an internal issue, such as a full inbox. If the recipient or an automated system (like an email client configured for automatic image loading) accesses the email during the brief window before the final bounce is processed and returned, an open event might be recorded. This often leads to confusion, as the sender receives a bounce notification but their tracking shows an open. Understanding this behavior is crucial for accurate email campaign analysis. You can find more information about how hard and soft bounces differ.
Impact on deliverability and how to manage bounces
Managing bounces effectively is paramount for maintaining a good sender reputation and ensuring high email deliverability. High bounce rates signal to Mailbox Providers (MBPs) that your list quality is poor or that you might be engaging in questionable sending practices, potentially leading to your emails being flagged as spam or even your IP address (or domain) being added to a blocklist (blacklist).
Monitoring your bounce rates and understanding the specific reasons behind each bounce type is crucial. For hard bounces, it's essential to remove those addresses from your mailing list immediately, as they will never be deliverable. For soft bounces, you might retry sending a few times, depending on the bounce reason, but persistent soft bounces should also lead to list cleaning.
Best practices for bounce management
Regular list hygiene: Implement regular list cleaning to remove invalid or inactive addresses.
Double opt-in: Use a double opt-in process for new subscribers to verify email addresses.
Monitor bounce rates: Keep an eye on your email bounce rate and aim to keep it below 2%.
Use reliable sending services: Choose an email service provider (ESP) with robust bounce handling.
Views from the trenches
Best practices
Actively monitor your bounce rates and categorize them into hard or soft bounces.
Set up automated systems to suppress hard-bounced addresses immediately.
For soft bounces, review the specific error codes to understand if retries are advisable.
Implement a robust email validation process at the point of data capture to reduce invalid addresses from entering your list.
Regularly cleanse your email lists of unengaged or repeatedly soft-bouncing contacts.
Common pitfalls
Ignoring bounce notifications or failing to categorize them, which can lead to continued sending to invalid addresses.
Not removing hard bounces from your list, harming your sender reputation and increasing future bounce rates.
Assuming all bounce messages mean the same thing, overlooking nuanced errors from different mailbox providers.
Over-retrying soft bounces without understanding the underlying temporary issue, wasting sending resources.
Failing to understand the difference between a bounced email and a read email, leading to skewed analytics.
Expert tips
Focus on domain reputation metrics within tools like Google Postmaster Tools for a broader view of bounce impacts.
Pay attention to the specific error messages and codes within bounce notifications; they often contain critical diagnostic information.
Recognize that different mailbox providers have varying bounce handling policies and delays, so consistent behavior across all recipients isn't always expected.
If an email bounces but shows as 'read,' consider the possibility of a security scanner pre-opening the email before a final delivery decision.
Leverage DMARC reports to identify sources of unauthorized email that could be contributing to unexpected bounces.
Expert view
Expert from Email Geeks says that receiving mail servers can be configured in various ways, leading to inconsistent bounce behaviors. Some servers might record a bounce and then a delivery, or vice versa, indicating an issue with how the receiving server is processing mail.
2019-02-04 - Email Geeks
Expert view
Expert from Email Geeks says that if recipients are on different domains, their email servers will likely behave differently. Some servers send immediate bounces, while others send asynchronous bounces after attempting internal mail delivery.
2019-02-05 - Email Geeks
Navigating email bounces and their complexities
The variations in email bounce notifications and the puzzling phenomenon of bounced emails being marked as read stem from the intricate and often inconsistent nature of email systems. Mail servers operate independently, leading to different bounce timings and formats based on their configurations and the type of delivery failure. The 'read' status for a bounced email is typically a consequence of automated security scans or a brief window of accessibility before a final delivery failure is reported.
