What is the current state of IPv6 adoption for email sending?

IPv6 adoption for email sending is growing, particularly for inbound mail at major providers like Outlook and Gmail. However, many ESPs still primarily send over IPv4, often using a dual-stack approach to support both protocols. The transition is slower than anticipated, mainly due to complexities in reputation management and legacy infrastructure.

What are the main challenges for email deliverability with IPv6?

Key challenges include managing IP reputation across the vast IPv6 address space, where blocklists often apply to large /64 blocks, and the ease with which spammers can rotate IPv6 addresses. This shifts the focus from individual IP reputation to a greater reliance on domain reputation .

Is email authentication as important for IPv6 as for IPv4?

Yes, robust email authentication via SPF, DKIM, and DMARC is critical for IPv6 email deliverability. Since IP reputation is less granular, domain authentication signals become paramount for mailbox providers to trust your emails.

Do I need to warm up my IPv6 addresses?

You should apply the same IP warming best practices to IPv6 ranges (like /64 blocks) as you would to IPv4 IPs. Gradually increase sending volume to build a positive reputation with mailbox providers.

Will switching to IPv6 automatically improve my email deliverability?

While native IPv6 connectivity might improve routing efficiency, it doesn't inherently guarantee better deliverability or bypass spam filters. Deliverability relies on a combination of factors, including content quality, sender reputation, and adherence to email authentication standards.

What is the current state of IPv6 for email sending and what are the challenges and best practices? - Technical - Email deliverability - Knowledge base

The internet's ongoing evolution from IPv4 to IPv6 has been a gradual, yet inevitable, process. While IPv6 offers a virtually limitless supply of IP addresses, a stark contrast to IPv4's dwindling pool, its adoption for email sending has progressed at a slower pace than some might have anticipated.

Many email service providers (ESPs) and businesses continue to rely heavily on IPv4, often facing challenges like the time it takes to secure dedicated IPv4 addresses. This reliance persists despite the clear benefits IPv6 brings, such as increased address space and potential for streamlined network management.

The transition isn't just about technical infrastructure, it also involves significant shifts in how email deliverability and sender reputation are managed. Understanding the current state, its inherent challenges, and the best practices for navigating this landscape is crucial for anyone involved in email operations.

The current landscape of IPv6 email sending

Today, a significant portion of inbound email traffic from major mailbox providers now leverages IPv6. This includes giants like

Google

and Microsoft (Outlook), which have been instrumental in pushing IPv6 adoption on the receiving end. While this is a positive sign, the volume of email originating from ESPs via IPv6 remains relatively low.

Estimates suggest that perhaps only 5-10% of total email traffic currently flows over IPv6. This percentage is heavily influenced by large senders and mailbox providers that have fully embraced the protocol. For most senders and ESPs, IPv4 is still the default and primary method for outgoing mail. Despite the lower adoption by ESPs, the industry is aware of the shift. The IPv6 deployment status remains a key discussion point in networking circles.

Many ESPs are considering, or have already implemented, a dual-stack approach, where they send mail over both IPv4 and IPv6, prioritizing IPv6 where supported. This strategy allows them to leverage the benefits of IPv6 while maintaining compatibility with systems that are still IPv4-only.

Feature	IPv4	IPv6
Address space	Limited (approx. 4.3 billion addresses)	Vast (340 trillion trillion trillion addresses)
Reputation management	Primarily IP-based, well-established blocklists	Shifting to domain-based; IP reputation for ranges (/64)
Configuration	Simpler, more familiar for legacy systems	More complex due to address length, but offers auto-configuration

Challenges and considerations for IPv6 email deliverability

The immense address space of IPv6 introduces unique challenges for managing email reputation. With so many possible IP addresses, the traditional method of tracking reputation at the individual IP address level becomes impractical. Instead, reputation is often assessed at a network prefix level, such as a /64 block, which is the smallest recommended allocation for end-users.

This larger block size means that a single bad actor or a compromised device within an IPv6 network segment can impact the deliverability of legitimate senders sharing that same prefix, leading to broad blocklist (or blacklist) entries. This can be particularly frustrating for senders who are otherwise adhering to best practices, but find themselves caught in a 'bad neighborhood' due to their provider's IP allocation policies.

Another significant challenge stems from the ease with which spammers and botnets can acquire and rotate IPv6 addresses. While setting up DNS for every single IP might seem time-consuming, automated systems can manage this with minimal effort, allowing malicious senders to quickly cycle through IPs and evade traditional IP-based filtering. This highlights why focusing solely on IP reputation, as has been common with IPv4, is increasingly insufficient for IPv6. The emphasis is shifting towards domain-based reputation and robust email authentication. You can learn more about why IPv6 emails go to spam.

IPv4 vs. IPv6 reputation management

The fundamental difference in IP allocation and scarcity between IPv4 and IPv6 profoundly impacts how email reputation is assessed and managed by Internet Service Providers (ISPs) and blocklist operators.

IPv4 reputation: Highly granular, often focusing on individual IP addresses or small blocks (e.g., /24). Blocklisting individual IPs is common.
Blocklist impact: Specific IP blocks are added to DNSBLs (DNS-based Blackhole Lists) or RBLs (Real-time Blackhole Lists).

Best practices for sending email over IPv6

While IPv6 introduces challenges, best practices exist to ensure good email deliverability. Foremost among these is the reinforcement of email authentication protocols. Since IP reputation becomes less precise with IPv6, Domain-based Message Authentication, Reporting, and Conformance (DMARC), Sender Policy Framework (SPF), and DomainKeys Identified Mail (DKIM) become even more critical for verifying sender identity. Mailbox providers increasingly rely on these signals to determine the legitimacy of an email.

Setting up SPF, DKIM, and DMARC correctly for your sending domains is non-negotiable. This includes ensuring proper domain alignment and managing your DNS records meticulously. A well-configured reverse DNS (PTR) record for your IPv6 sending IPs is also essential, even if reputation signals are broader. It provides a fundamental layer of trust and verification for receiving mail servers. For more details on reverse DNS resolution, consult our guide.

Just like with IPv4, warming up your IPv6 IP ranges is crucial to building a positive sending reputation. Even with a /64 allocation, gradually increasing your sending volume from these new IPs allows mailbox providers to assess your sending behavior and build trust over time. Neglecting IP warming can lead to immediate deliverability issues, including being blocklisted, regardless of the protocol. It's also vital to monitor your IPv6 blocklist status to quickly identify and address any reputation issues. You can check your IP status using a blocklist checker.

Example AAAA record for IPv6

AAAA RecordDNS

yourdomain.com IN AAAA 2001:0db8:85a3:0000:0000:8a2e:0370:7334

This DNS record maps your domain to an IPv6 address, enabling receiving servers to find your mail server via IPv6. Similarly, PTR records for IPv6 (often called reverse DNS for IPv6) are vital for mail server verification.

Views from the trenches

While IPv6 offers a vast address space and improved efficiency, its adoption in email sending, particularly by ESPs, is still evolving. The shift from IPv4's scarcity to IPv6's abundance brings new considerations for reputation management. This is especially true for IP-based filtering, which must adapt to larger network blocks and the automated rotation capabilities of malicious actors.

The industry consensus leans heavily towards a dual-stack approach, combining both IPv4 and IPv6, and a stronger emphasis on domain-based reputation signals rather than solely IP reputation. Mailbox providers, such as Google, already accept a significant portion of their inbound mail over IPv6, even if ESPs are slower to fully transition. This underscores the need for senders to prioritize robust email authentication (SPF, DKIM, DMARC) and diligent IP warming, regardless of the IP version in use.

Best practices

Actively implement and monitor SPF, DKIM, and DMARC for all sending domains and ensure proper alignment for enhanced trustworthiness.

If you manage your own IPs, obtain your own IPv4 range (e.g., a /24) for greater flexibility and control over your sending reputation.

Prioritize sending as much legitimate mail as possible over IPv6, with IPv4 as a fallback, especially to major mailbox providers.

Warm up IPv6 /64 ranges diligently and monitor their performance as you would with IPv4 addresses.

Maintain accurate and properly configured reverse DNS (PTR) records for all IPv6 sending IPs.

Common pitfalls

Relying solely on IP-based reputation for IPv6, as it's less granular and more susceptible to neighborhood issues.

Using providers that assign IPv6 blocks smaller than /64, which can lead to disproportionate blocklisting by major blocklist operators.

Neglecting proper email authentication (SPF, DKIM, DMARC) with the assumption that IPv6 alone will improve deliverability.

Failing to warm up new IPv6 IP ranges, leading to poor reputation and increased spam filtering.

Assuming that IPv6 provides an automatic bypass for poor sending practices or content quality.

Expert tips

Focus on strong domain reputation: With IPv6's vastness, domain reputation and authentication signals are more critical for deliverability.

Adopt a dual-stack strategy: Implement both IPv4 and IPv6 sending capabilities, gradually increasing IPv6 volume.

Understand ASN-based reputation: Some mailbox providers (like Microsoft) may use ASN-level reputation, impacting large ranges of IPs.

Automate DNS setup: Ensure your systems can quickly and automatically manage DNS records for IPv6 IPs to counteract spammers' automation.

Choose providers carefully: Select ESPs or hosting providers that adhere to IPv6 best practices for IP allocation and management.

Expert view

Expert from Email Geeks says IPv6 works well at many mailbox providers, but modern authentication is a prerequisite for good deliverability.

2024-03-19 - Email Geeks

Expert view

Expert from Email Geeks says trying to be an IPv6-only ESP is not advisable, as not all mailbox providers fully support it yet.

2024-03-19 - Email Geeks

The future of email with IPv6

The transition to IPv6 for email sending is ongoing, characterized by slow but steady adoption, especially among large email ecosystem players. While challenges remain, particularly concerning reputation management in a vastly expanded address space and dealing with improper IP allocations, the path forward is clear.

For senders, the key is to prioritize fundamental deliverability practices, chief among them strong email authentication (SPF, DKIM, DMARC), meticulous DNS management, and consistent IP warming. By embracing a dual-stack approach and understanding that domain reputation will increasingly outweigh individual IP reputation, businesses can ensure their messages reach the inbox, regardless of the underlying internet protocol. This proactive approach helps mitigate risks and positions senders for success in the evolving email landscape.