Which mail box providers reliably send async bounces instead of synchronous bounces?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 Jul 2025
Updated 17 Aug 2025
6 min read
Email bounces are a critical aspect of deliverability, signaling that a message couldn't reach its intended recipient. Understanding the different types of bounces, particularly synchronous and asynchronous, is crucial for maintaining a healthy sender reputation and optimizing email campaigns.
Synchronous bounces happen instantly during the SMTP transaction, providing immediate feedback that an email was rejected. Asynchronous bounces, on the other hand, occur after the receiving server has initially accepted the email but later determines it cannot deliver it. These are typically reported via a Non-Delivery Report (NDR) or Delivery Status Notification (DSN).
The question of which mailbox providers reliably send asynchronous bounces is frequently asked, especially when trying to ensure comprehensive bounce processing. Most major providers primarily favor synchronous bounces for efficiency and to mitigate issues like backscatter.
Understanding synchronous and asynchronous bounces
Synchronous bounces are the most common form of email rejection. When you send an email, and the recipient's mail server immediately declines it during the SMTP handshake, that's a synchronous bounce. This direct feedback is highly beneficial because it provides immediate insight into delivery failures, allowing for quick suppression of invalid addresses.
Common reasons for synchronous bounces include an invalid recipient address, a full mailbox, or temporary server issues. The mail server returns an SMTP error code directly to the sending server, such as 550 User unknown. Because the rejection happens in real-time, email service providers (ESPs) can process these bounces efficiently and update sender lists promptly.
Asynchronous bounces, also known as delayed bounces, occur after the initial acceptance of the email by the recipient server. The server might accept the message for delivery, only to discover later that it cannot be delivered. This could be due to a mailbox being over quota, an internal routing issue, or even a server-side spam filter identifying the message as problematic after initial acceptance.
These bounces are typically communicated back to the sender via a separate email, usually a Non-Delivery Report (NDR) or Delivery Status Notification (DSN). The delay in receiving these notifications can make them more challenging to process and manage in real-time for list hygiene.
Synchronous bounces
Occurrence: During the SMTP connection, typically within seconds.
Examples: Invalid email address, domain not found, recipient server offline.
Mailbox provider tendencies for bounce types
From a deliverability perspective, most mailbox providers (MBPs) and email sending platforms are designed to handle synchronous bounces preferentially. The immediate feedback allows for efficient resource management and better compliance with sender reputation best practices.
Major MBPs, like Gmail and Yahoo, largely rely on synchronous bounces for invalid or problematic recipients. This minimizes the back-and-forth communication, reducing bandwidth and server load. As Twilio SendGrid documentation notes, the vast majority of bounces are synchronous.
While most providers aim for synchronous rejections, asynchronous bounces can still occur. These often arise in more complex email infrastructures, particularly in Business-to-Business (B2B) scenarios where mail might traverse multiple layers of security appliances (e.g., Proofpoint, Mimecast) before reaching the final inbox. A delivery failure at a later hop in this chain can trigger an asynchronous bounce.
Specific providers are not known to reliably send asynchronous bounces as their primary method. However, issues like a mailbox exceeding its quota might cause a delay before the bounce is issued, making it asynchronous. For example, a Hotmail mailbox with a full OneDrive folder has been observed to sometimes trigger an asynchronous bounce due to storage limitations.
Why asynchronous bounces are rare
Synchronous bounce processing
Mailbox providers and senders generally prefer synchronous bounces. They are simple to handle because the rejection happens during the initial SMTP conversation. This allows for immediate removal of invalid addresses from mailing lists.
From a system perspective, synchronous bounces are resource-efficient. The server doesn't accept the email only to send it back later, which saves bandwidth and processing power. This efficiency is a primary reason why most providers have moved towards maximizing synchronous rejections.
Historically, asynchronous bounces contributed to the backscatter problem. Backscatter occurs when a mail server generates an NDR for an email sent to a nonexistent address, but the NDR is sent to a forged sender address. Spammers exploited this by sending emails with forged sender addresses, causing legitimate domains to receive these unwanted bounce messages.
To combat backscatter and reduce abuse, many MBPs tightened their policies and moved to reject questionable or invalid emails synchronously. This shift significantly reduced the prevalence of asynchronous bounces from primary providers.
Even with these efforts, asynchronous bounces are sometimes unavoidable. They can happen due to recipient mailbox full issues, internal mail system failures, or misconfigurations within a recipient's complex email infrastructure. For example, a DMARC policy failing after multiple hops can result in a delayed rejection.
Managing and testing asynchronous bounces
Since asynchronous bounces are less common and often delayed, they pose unique challenges for senders. The delayed feedback means that invalid email addresses may remain on active sending lists longer, potentially impacting your sender reputation and deliverability metrics.
To effectively manage asynchronous bounces, ESPs must have robust systems to receive, parse, and act upon NDRs. This often involves setting up specific mailboxes to collect these bounce messages and then automating the process of identifying the original recipient and suppressing them. Without this, you might continue sending to dead addresses, which can lead to poor inbox placement and even blocklisting.
For testing purposes, it's generally not feasible to rely on a specific MBP to reliably send asynchronous bounces. Most will prioritize synchronous rejections. If you need to simulate an asynchronous bounce for testing your system's bounce processing, you'd typically need to manually craft and inject one into your Mail Transfer Agent (MTA).
Example of an asynchronous bounce message (NDR)text
From MAILER-DAEMON@example.com Tue Dec 17 07:14:26 2024
Date: Tue, 17 Dec 2024 07:14:26 -0500
From: MAILER-DAEMON@example.com (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: reports@yourdomain.com
This is the mail system at host example.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<recipient@bounceddomain.com>: host mx.bounceddomain.com[192.0.2.1]
said: 550 5.1.1 <recipient@bounceddomain.com>: Recipient address rejected: User unknown (in reply to RCPT TO command)
--original message--
Subject: Your Newsletter
From: sender@yourdomain.com
To: recipient@bounceddomain.com
Date: Tue, 17 Dec 2024 07:10:00 -0500
Views from the trenches
Best practices
Implement a robust bounce processing system that can handle both synchronous and asynchronous bounce types to maintain a clean mailing list.
Monitor your DMARC reports closely, as they can sometimes reveal asynchronous bounce patterns, especially if alignment issues cause delayed rejections.
Regularly audit your email lists to remove inactive or problematic addresses before they trigger bounces, proactively improving deliverability.
Ensure your ESP or internal system properly classifies bounce codes (hard vs. soft) to prevent sending to permanently invalid addresses.
Common pitfalls
Ignoring asynchronous bounces can lead to continued sending to invalid addresses, negatively impacting sender reputation and potentially leading to blocklist (or blacklist) listings.
Not having a dedicated bounce processing mailbox or automated system for handling NDRs can result in manual, time-consuming efforts.
Over-reliance on synchronous bounce data alone, missing the insights provided by delayed rejections from complex mail infrastructures.
Failing to understand the full path of an email in B2B environments, where multiple security layers might introduce delayed bounce scenarios.
Expert tips
While specific mailbox providers don't reliably send async bounces by design, they do occur, particularly with full mailboxes or complex B2B setups.
The industry largely moved away from extensive async bounces to combat backscatter and optimize resources, making them genuinely rare from major providers.
If you need to test async bounce processing, consider injecting a crafted bounce message directly into your MTA rather than relying on a specific provider to generate one.
Pay close attention to bounced emails that come from intermediate mail servers or spam filtering services, as these are more likely to be asynchronous.
Marketer view
Marketer from Email Geeks says most mailbox providers would prefer to send synchronous bounces whenever possible because it involves less bandwidth and resource consumption for them.
July 9, 2024 - Email Geeks
Marketer view
Marketer from Email Geeks says that while asynchronous bounces are rare, they are sometimes unavoidable, especially when badly implemented forwarding or spam filtering services are involved.
July 9, 2024 - Email Geeks
Key takeaways for senders
While no specific mailbox provider reliably sends only or mostly asynchronous bounces, understanding both types of rejections is essential for effective email deliverability. The industry's lean towards synchronous bounces is a strategic move for efficiency and security.
For senders, the focus should remain on maintaining a clean email list, regularly processing all bounce types, and utilizing tools and strategies that help boost deliverability rates. Being prepared to handle the rarer asynchronous bounces ensures no potential recipients are missed and your sender reputation remains strong.
Gmail and 
Yahoo, largely rely on synchronous bounces for invalid or problematic recipients. This minimizes the back-and-forth communication, reducing bandwidth and server load. As Twilio SendGrid documentation notes, the vast majority of bounces are synchronous.
Hotmail mailbox with a full OneDrive folder has been observed to sometimes trigger an asynchronous bounce due to storage limitations.
