IPv4 vs IPv6 for email deliverability in 2021

Victor Minev Aug 31 2021 Share

Why IPv6 at all?

The reason we’re talking about IPv6 addresses in the first place is the fact that IPv4 addresses ran out some time ago, and whatever second hand IPv4 addresses remain available to rent or buy will continue to grow in cost.

IPv6 solves one big problem, plus a few smaller ones, but in the short term introduces many more. There are quite a few differences between the two protocol versions — here we’ll focus on what IPv6 means for emails and potential impacts on deliverability.

How IPv6 affects email

As we’ve previously defined it email deliverability addresses where an email winds up after it’s accepted for delivery. Many factors affect this, such as:

  • Email authentication
  • Email content
  • Sender reputation

None of those however are IPv6-specific, and the newer version of the protocol doesn’t bring any specific functionality related to email deliverability that’s not already available using IPv4. There are however pitfalls which are unique to IPv6 related to both email deliverability and delivery:

  • Dual-stacked email servers
    The official specification (RFC 6724) suggests a preference for IPv6 over IPv4 in email communication between servers where the destination has both protocols configured. Which protocol to use first is decided independently by each mailops manager, and many have already taken this step. That means once you’ve enabled IPv6 for your mailstack it needs to work properly, else you’re better off not using it at all. 

Additionally you’ll need to test that your fallback to an IPv4 address is operational for those remote hosts which use such a mechanism. That protects you from problems in both your own IPv6 stack, and the setup of remote hosts, some of whom overambitiously prefer IPv6 without having first ironed out the kinks.

  • Stricter requirements
    Some providers like Gmail have stricter requirements for emails coming from IPv6. That’s not to say emails sent over v4 IPs without a properly set up PTR and authentication will pass their filters, but it’s one thing to list those actions as recommended guidelines, as they do for IPv4, and another to enforce them as firm requirements, which they do for IPv6.
  • Blacklisting-related issues
    The IPv6 space is significantly larger and there are recommendations for assigning many different addresses (e.g. a /64 block) to each individual user. This creates a lot of leeway for bad actors to use up and throw away more addresses, thereby recycling their identity before it can catch up with them, and it has also created some difficulties for blacklisting organizations, such as RBL maintainers, who’re trying to catch up with them.


IPv6 has a lot of advantages, the main one being – the space is so much larger than the IPv4 one. The newer version of the protocol introduces a lot of breathing space for the faster development of the internet as an overall, including email. There are also a lot of enhancements related to the way it’s set up, auto-configuration, ease of use, etc. 

That being said, no specific benefits of IPv6 over IPv4 in terms of email deliverability can be outlined. The most important factors that have an effect on this aspect of email exist regardless of the version of the protocol we’re sending through. 

Nevertheless, as explored earlier we’ve already run out of pristine, unused IPv4 addresses. Yes, a used IP address can be recycled in terms of email and used by another organization, but that’s not a scalable way going forward. Though the adoption of IPv6 around the globe is currently proceeding slowly, it’s the only foundation for future Internet, and therefore email, growth.

Final thoughts

Instead of looking at IPv6 simply as a replacement of IPv4, for now you may view it as an optional step towards future-proofing and scalability of your mailops. While it’s fine for now to keep using IPv4 alone, it’s definitely worth exploring the implementation of a dual solution with IPv6, if you’re ready to reliably maintain it, for the sake of stability in your future operations.

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