What’s the difference between IPv4 and IPv6 in email delivery?

If you've ever wondered why your ESP sometimes assigns you a long, cryptic string of an IP address instead of the familiar four-number format, you've stumbled into the IPv4 vs. IPv6 story. It matters more for email than most people realize.

IPv4 is the older system. Every IP looks like four numbers separated by dots, like 192.168.1.1. There are roughly 4 billion possible addresses, and the internet ran out of fresh ones years ago. Most email today still flows over IPv4, which means the IP reputation systems that mailbox providers rely on are mature and well-tested for it.

IPv6 uses a much longer format, like 2001:0db8:85a3::8a2e:0370:7334, and offers an astronomically large address pool. Practically speaking, there are enough IPv6 addresses to assign one to every grain of sand on Earth and still have room left over. That sounds like a win. The catch is that IPv6 reputation infrastructure is newer, thinner, and less consistent across receiving domains.

So what does this mean for your emails getting delivered?

• Reputation doesn't transfer. If you've built solid sender reputation on your IPv4 address over years, none of that carries over to your IPv6 address. They're tracked separately. Some senders get burned by this when they add IPv6 sending without realizing their new IPv6 address starts with zero history.
• SPF needs to cover both. Your SPF record has to explicitly include both your IPv4 ranges (using the ip4: mechanism) and your IPv6 ranges (using ip6:). If you send from an IPv6 address that isn't listed in your SPF record, receiving servers will treat it as unauthorized.
• PTR records matter for both. A PTR record is the reverse DNS lookup that maps an IP address back to a hostname. Mailbox providers like Gmail and Outlook check these as part of spam filtering. You need a properly configured PTR record for every IPv4 and IPv6 address you send from.
• Some receivers handle IPv6 differently. A handful of mailbox providers still have inconsistent or limited support for IPv6 delivery. This means a message sent over IPv6 might be filtered more aggressively, or not accepted at all, compared to the same message sent over IPv4.

The practical advice for most senders is this. IPv4 is still your primary workhorse. If you're using a good ESP, they're managing your IP assignments and most of this happens behind the scenes. But if you run your own mail infrastructure, or if your ESP has started assigning you IPv6 addresses, it's worth double-checking that your SPF record includes both protocols and that PTR records are in place for every sending IP.

Not sure if your SPF is covering all your sending addresses? You can check it in about 30 seconds with our free SPF checker. If something looks off, the SOS hotline is free and we'll walk you through it.