What is the current state of IPv6 for email sending and what are the challenges and best practices?

Key findings

• Limited ESP adoption: Few ESPs are currently sending substantial volumes of email over IPv6 for their clients, despite the clear long-term benefits of the protocol.
• Mailbox provider acceptance: Major mailbox providers, including Google and Microsoft (Outlook), actively accept and process IPv6 email, with it constituting a notable percentage of their daily inbound traffic.
• Authentication criticality: Robust email authentication (SPF, DKIM, DMARC) is even more critical for IPv6 email deliverability than for IPv4, as traditional IP-based reputation models are less effective.
• Reputation management complexities: The sheer scale of IPv6 addresses (a /64 block alone contains more IPs than all of IPv4) makes traditional single-IP reputation tracking difficult for blocklists and filtering systems. This often leads to reputation being assessed on larger network blocks or domains.

Key considerations

• Strategic IPv6 adoption: ESPs with foresight should strategically plan for increasing IPv6 mail sending, potentially utilizing dedicated IPv6 addresses with a smaller IPv4 fallback pool.
• Standard IP allocation: Adhering to standard IPv6 address allocation sizes, such as a /64, is crucial. Receiving smaller, non-standard allocations can lead to entire blocks being blacklisted due to spam originating from other users within the same allocation.
• Domain-centric reputation: The challenges with IP-based reputation in IPv6 environments further underscore the shift towards focusing on domain reputation, alongside Autonomous System Number (ASN) reputation and content analysis. Learn more about how to improve email sending practices and domain reputation.
• Warming process still applies: Even with IPv6, warming a new IP address range (/64) and associated domains is necessary to establish a positive sender reputation. See the status of IPv6 deployment to understand current progress and obstacles.

Key opinions

• IPv4 scarcity is real: The process of obtaining additional dedicated IPv4 addresses is increasingly lengthy and challenging, limiting scaling opportunities.
• ESPs are lagging: Many ESPs have not yet integrated robust IPv6 sending options for their clients, forcing marketers to rely on IPv4.
• Desire for IPv6: Marketers are eager for IPv6 to become a more viable and widely adopted solution for high-volume email sending to circumvent IPv4 limitations.
• Authentication is non-negotiable: Regardless of the IP version, strong email authentication (SPF, DKIM, DMARC) is seen as essential for successful inbox placement.

Key considerations

• Proactive IP management: It is prudent to anticipate future IP needs and address them before encountering rate limits or delivery issues. Consider the best practices for setting email sending speed.
• Consider BYO IP: For greater flexibility and control, explore options for bringing your own IPv4 range if your ESP supports it, allowing you to retain your IP assets if you switch providers.
• Pilot IPv6 sending: If an ESP offers IPv6 sending, start with small-scale testing to observe performance and troubleshoot any potential deliverability issues.
• Spam filtering insights: IPv6 can offer more precise IP tracking for spam detection. A mySMTP BLOG article highlights how this can aid in blocking spammers effectively.

Key opinions

• IPv6 functionality: IPv6 mail delivery generally functions well at many major mailbox providers, given proper configuration and authentication.
• ESPs' slow embrace: Despite IPv4 scarcity, most ESPs are not widely adopting or offering IPv6 for their customer's sending pools.
• Reputation challenges: The massive scale of IPv6 addresses makes traditional single-IP based reputation difficult, leading filtering systems to focus on larger address blocks or domain-level reputation.
• Problematic IP allocations: Some hosting providers issue IPv6 allocations smaller than the recommended /64, which can result in good senders being blocklisted due to other bad actors within the same large block.

Key considerations

• Avoid IPv6-only strategies: Due to inconsistent IPv6 support across all mailbox providers, attempting to operate as an IPv6-only ESP is not advisable in the current landscape.
• Provider choice matters: Choose hosting and ESPs that adhere to standard IPv6 allocation practices (/64 minimum) to mitigate the risk of broad blocklisting. Review best practices for IP network configuration.
• Domain and content priority: Email filtering and reputation are increasingly tied to domain trustworthiness and content quality, diminishing the relative importance of individual IP address reputation. See the IPv6 paradox for more on slow adoption.
• Spammer adaptability: Spammers can easily automate domain and IP rotation, making it less effective to rely solely on IP-based blocklisting for long-term protection.

Key findings

• Expansive address space: IPv6 provides a virtually inexhaustible supply of IP addresses, a critical solution to the global exhaustion of IPv4 addresses.
• Enhanced efficiency: With a simplified packet header and more efficient routing, IPv6 is designed to offer faster performance and reduced latency.
• Elimination of NAT: IPv6 eliminates the need for Network Address Translation, simplifying network architecture and potentially improving end-to-end connectivity.
• Security and deployment challenges: Despite its benefits, the migration to IPv6 faces challenges related to new security vulnerabilities and the inherent complexities of large-scale deployment.

Key considerations

• Comprehensive DNS configuration: Proper configuration of AAAA records and reverse DNS (rDNS) for IPv6 addresses is crucial for email authentication and deliverability. Learn more about reverse DNS resolution best practices.
• Strategic transition planning: Organizations must develop robust strategies for transitioning to IPv6, often employing dual-stack environments to ensure compatibility with both protocols during migration. RFC 6589 highlights challenges when transitioning content to IPv6.
• Addressing security concerns: Awareness and mitigation of new security vulnerabilities specific to IPv6 are essential for maintaining a secure email infrastructure.
• Adherence to allocation standards: Documentation consistently recommends that the smallest IPv6 allocation should be a /64, a guideline critical for proper network function and to avoid blocklisting issues.