Are asynchronous bounces back in email marketing and where will the DKIM2 discussion take place?
Michael Ko
Co-founder & CEO, Suped
Published 11 Jul 2025
Updated 17 Aug 2025
8 min read
Email marketing is a constantly evolving landscape, and just when we think we have a handle on all its complexities, new challenges or resurfacing issues emerge. Two topics currently sparking considerable discussion within the email community are the apparent comeback of asynchronous bounces and the ongoing dialogue surrounding DKIM2. Both have significant implications for how we manage email deliverability and authentication going forward.
Understanding these shifts is crucial for maintaining strong sender reputations and ensuring messages reach the inbox. This article will delve into what asynchronous bounces mean for email marketers today and explore where the important discussions around the next iteration of DKIM are taking place.
Asynchronous bounces occur when an email is initially accepted by the recipient’s mail server, but a delivery failure notification is sent back to the sender much later, sometimes hours or even days later. This contrasts with synchronous bounces, which are immediate rejections at the time of sending. For a long time, the trend has been towards more synchronous rejections, making asynchronous bounces less common. However, recent observations suggest they are becoming more prevalent.
This resurgence can be attributed to several factors. Sometimes, it’s due to the recipient's mailbox being full, an invalid recipient address, or even mail forwarding issues where the ultimate destination rejects the message after the initial server accepts it. Mailbox providers might also accept mail initially for further scanning, only to bounce it later if it fails security checks or internal policy rules. Such delayed notifications pose unique challenges for email marketers.
The impact of asynchronous bounces on email marketing and sender reputation is significant. Since the bounce notification is delayed, it can take longer to identify and remove invalid addresses from your lists, potentially leading to continued sending to problematic recipients. This can negatively affect your sender score and increase your overall bounce rate, signaling to Internet Service Providers (ISPs) that your sending practices might be poor. Understanding the nuances of different bounce types is critical for effective list hygiene.Classifying and handling different bounce types is a crucial step in maintaining a healthy email program.
Properly classifying and managing these delayed bounce responses is key to mitigating their negative impact. It requires robust bounce processing systems that can effectively parse Non-Delivery Reports (NDRs) and update subscriber statuses in real-time. Ignoring them can lead to being placed on email blacklists (or blocklists), impacting your deliverability even further.
Best practices for asynchronous bounce handling
Monitor NDRs closely: Ensure your systems are configured to receive and parse Non-Delivery Reports, often sent to the Return-Path address.
Categorize bounces: Distinguish between soft and hard asynchronous bounces to inform your suppression strategy. While a hard bounce indicates a permanent failure, a soft bounce might just be temporary.
Implement timely suppression: Promptly remove addresses that generate hard bounces, even if delayed, to protect your sender reputation.
Review logs: Regularly check your email sending logs for signs of delayed bounces or unusual delivery patterns.
The challenge of tracking and managing asynchronous bounces
Identifying and processing asynchronous bounces presents a unique challenge for email service providers (ESPs) and senders. Unlike synchronous bounces, which provide immediate feedback, these delayed responses require continuous monitoring of the return-path (or SMTP Mail From) address, where NDRs are typically sent. If your infrastructure isn't designed to efficiently parse these late bounce messages, you might be unknowingly continuing to send to invalid or problematic addresses.
The Return-Path header plays a critical role in this process. It specifies where bounce messages should be sent. If this isn't correctly configured or monitored, you risk missing vital feedback about your email delivery. Properly managing Return-Path and SMTP Mail From headers ensures that bounce information is directed to the right place for processing.
The impact on sender reputation can be subtle but damaging. ISPs evaluate sender reputation partly based on bounce rates. High bounce rates, even if composed of delayed bounces, signal poor list hygiene or problematic sending practices. This can lead to your emails being filtered into spam folders, throttled, or even blocklisted (blacklisted). Regularly checking your bounce statistics and delving into the causes, including asynchronous bounces, is essential.
Delayed notification (hours or days) after initial acceptance.
Feedback Source
Receiving mail server responds during the sending attempt.
Non-Delivery Report (NDR) or DSN sent to the Return-Path.
Common Causes
Invalid recipient, server unavailable.
Full mailbox, forwarding issues, content filtering by Google or other providers.
Impact on Data
Easier to immediately remove invalid addresses.
Delayed data updates, potential for continued sending to bad addresses.
DKIM2: The next evolution in email authentication
DomainKeys Identified Mail (DKIM) has been a cornerstone of email authentication for years, helping to verify the sender of an email and ensure its integrity. However, as email threats become more sophisticated, the need for enhanced authentication mechanisms grows. This is where DKIM2 comes into play, representing the anticipated next iteration of the DKIM standard. While details are still emerging, DKIM2 aims to address existing limitations and strengthen email security even further.
The primary objectives of DKIM2 are likely to include improving cryptographic strength, enhancing resistance to various attack vectors, and potentially streamlining implementation. Current DKIM relies on RSA key pairs, and with advancements in computing power, stronger algorithms are always being explored. DKIM2 could introduce new signing algorithms or more robust key management practices, making it harder for malicious actors to forge emails. For more on the future, consider new email authentication requirements.
The necessity for DKIM2 stems from the continuous arms race between email senders and malicious actors. As DMARC (Domain-based Message Authentication, Reporting, and Conformance) becomes more widely adopted and strictly enforced by major mailbox providers, the underlying authentication protocols like SPF (Sender Policy Framework) and DKIM must evolve to meet new security demands. Stronger authentication directly translates to better inbox placement and a reduced risk of phishing and spoofing attacks. Understanding how DKIM impacts domain reputation and email deliverability is key.
While specific syntax for DKIM2 is still under development, it will likely involve new tags or modifications to the existing DKIM record structure. Here’s an example of how a DKIM record might look, anticipating some future changes, though the exact specifications for DKIM2 are not yet finalized:
The DKIM2 discussion: Where it's happening and who's involved
The evolution of email authentication protocols like DKIM is a collaborative effort involving experts from across the industry. Discussions about DKIM2 are primarily taking place within specialized working groups and mailing lists of the Internet Engineering Task Force (IETF). This is the same body responsible for many internet standards, including the original DKIM specification.
Specifically, these discussions are often found on the existing IETF DKIM mailing list (ietf-dkim@ietf.org). While some initial conceptual discussions might occur in private forums, the formal standardization process, which leads to RFCs (Request for Comments) and official adoption, requires open public review and input on these lists. This ensures transparency and allows a broad range of stakeholders to contribute to the protocol's development.
For email marketers, deliverability professionals, and anyone invested in email security, monitoring these discussions is vital. Early insight into proposed changes for DKIM2 can help you prepare your infrastructure and strategies for future authentication requirements. It also provides an opportunity to contribute to the future of email, ensuring that new standards meet the practical needs of the email ecosystem. Keeping up-to-date with the benefits of DKIM v2 is important.
Engaging with these technical discussions can seem daunting, but it's essential for understanding the direction of email authentication. The IETF mailing lists are public archives, offering a wealth of information on proposed changes, technical challenges, and community consensus.
Current DKIM
Algorithm: Primarily RSA. While still strong, cryptographic best practices evolve.
Header Canonicalization: Relies on fixed canonicalization methods (simple/relaxed), which can sometimes break signatures if headers are modified.
Key Length: Often 1024-bit, though 2048-bit is becoming more common.
Deployment Complexity: Requires careful management of DNS records and private keys.
Anticipated DKIM2
Algorithm: Potential for stronger, more modern cryptographic algorithms like EdDSA (Ed25519).
Header Canonicalization: May introduce more flexible or robust canonicalization methods to better handle header modifications by intermediaries.
Key Length: Focus on higher key lengths and more secure key management principles.
Deployment Complexity: Aims to provide clearer guidelines and potentially automated tools for easier setup and management.
Navigating the evolving email landscape
The email landscape continues to evolve, bringing both new challenges and advancements. Asynchronous bounces, while not entirely new, appear to be a re-emerging factor that demands careful attention for optimal deliverability. Simultaneously, the ongoing discussions around DKIM2 highlight the industry's commitment to continually strengthening email authentication against sophisticated threats. Staying informed about both these trends is paramount for any email marketer or deliverability professional.
Proactive monitoring of bounce rates, diligent list hygiene, and a keen eye on developments in authentication protocols like DKIM2 will be key to navigating the future of email marketing successfully. These elements collectively contribute to a robust email program, ensuring your messages reliably reach their intended audience.
Views from the trenches
Best practices
Actively monitor your bounce logs for delayed NDRs to identify and suppress problematic addresses.
Segment your email lists based on engagement and bounce history to minimize risks from low-quality recipients.
Stay updated on IETF mailing list discussions regarding DKIM2 for early insights into future authentication standards.
Ensure your DMARC reporting is configured to provide visibility into authentication failures, including those related to DKIM.
Common pitfalls
Ignoring asynchronous bounces, leading to sustained sending to invalid addresses and reputation damage.
Relying solely on synchronous bounce feedback, missing the full picture of delivery issues.
Failing to update DKIM records as cryptographic best practices evolve, potentially weakening authentication.
Not engaging with the broader email community to understand emerging threats and protocol changes.
Expert tips
Consider using dedicated bounce processing services to efficiently handle and categorize all bounce types.
Implement a feedback loop strategy with major mailbox providers to receive timely spam and bounce reports.
Regularly audit your DNS records for SPF, DKIM, and DMARC to ensure they are correctly configured and aligned.
Leverage DMARC aggregate reports to detect trends in authentication failures, which can inform DKIM optimization.
Marketer view
Marketer from Email Geeks says asynchronous bounces are seeing a resurgence.
2024-11-05 - Email Geeks
Marketer view
Marketer from Email Geeks shares that based on their experience with Comcast postmaster boxes, asynchronous bounces have always been present.
Google or other providers.
