Has anyone had tried to run an IPv6-only SMTP engine? Pretty much everybody with any sense has IPv6 configured for major front-end servers. I was curious if anyone had tried to run an IPv6-only MTA and received any connection errors.

Is IPv6-only a viable solution yet? Can I expect a few lingering connection issues? Or did a magic fairy come down on the internet and made IPv6-to-IPv4 on port 25 work like magic on a direct connection?

  • 3
    "Is IPv6 only a viable solution yet?" What's your definition of "viable"? Regarding your last sentence - v4 and v6 are fundamentally incompatible. A v4 system cannot communicate with a v6 system without some intermediary router/NAT/proxy/etc.
    – EEAA
    Apr 17, 2016 at 14:14
  • I would imagine a 5% message transaction loss to be pretty horrid and about what I saw when I tried this 4 or 5 years ago.
    – joe
    Apr 17, 2016 at 14:16
  • 1
    "The IPv6 designers made a fundamental conceptual mistake: they designed the IPv6 address space as an alternative to the IPv4 address space, rather than an extension to the IPv4 address space. " cr.yp.to/djbdns/ipv6mess.html
    – kubanczyk
    Apr 18, 2016 at 7:26
  • 5
    @kubanczyk i don't see how you could of extended IPv4 address space without making a model that would be inherently incompatible with it? was there a better alternative? I feel the solutions given on that web-page only move the problem around, not fix it. it would still be down to everyone else to support it...
    – James T
    Apr 18, 2016 at 8:31
  • 1
    @kubanczyk to be clear there was no "fundamental mistake" since noone working on the project ever intended v4 for anything other than experimental use. Unlike v4 v6 is actually intended and designed for public use.
    – Jim B
    Apr 24, 2016 at 23:36

3 Answers 3


Short answer: it will work, technically, but you will have lots of undeliverable mail.

Long answer: Take your SMTP logs. Sed out all the domain names you send mail to. Check if they have IPv6 DNS and MX. Once you get 100% (you won't, not anytime this decade), then you can try if the IPv6 IPs actually work.

I don't have any interesting production logs at hand (those I do have don't have enough domains to be of interest), but I took a list of domains offering free e-mail services from https://gist.github.com/tbrianjones/5992856

Out of the 536 first, 173 did not seem to have any MX resolving to an IP, 7 had MXs resolving to IPv4 and IPv6 MX addresses, and the remaining 356 had only IPv4 MXs. Out of domains having MXs, that is less than two percent OK, even before actually trying the IPv6 address to see if it works. Even admitting that the domains in the list are not in any sense the majority of Internet e-mail domains, I do not think that is enough for running a mail server that you actually expect to use.

EDIT: since the 536 alphabetically first of a random list of over 3600 free e-mail providers is not very representative, I've checked a few big-name domains, and here are those that did not have IPv6 MXs (remember IPv6-accessible DNS would also be needed):

  • microsoft.com / hotmail.com / outlook.com
  • mail.com
  • gmx.net
  • icloud.com / mac.com
  • comcast.com
  • inbox.com
  • zoho.com
  • aol.com
  • orange.fr
  • twitter.com

Do you want to register a domain?

  • godaddy.com
  • networksolutions.com
  • registrar.com

Or . . . do you want mail from this site?

  • stackexchange.com

(Of course) gmail.com and google.com have IPv6, and so does Facebook.com.

For those who are interested, I used an ancestor to this line of bash script:

for i in $(cat domains.txt) ; do
  echo $(
    echo $i
    echo \;
    for j in $(dig +short mx $i) ; do
      dig +short a $j
      dig +short aaaa $i         
    done \
    | sed -r -e 's/[^;:\.]//g' \
             -e 's/^:+$/v6/'  \
             -e 's/^\.+$/v4/' \
    | sort -u
done \
| sed 's/ v4 v6/ v4+v6/' \
| sed -r 's/^([^;]+); *([^;]*)$/\2;\1/' \
| sed 's/^;/none;/' \
| sort '-t;' -k 1,1 \
| tr ';' '\t'

It's certainly improvable, but most of the bizarre things are to make the output prettier.

  • 2
    I think very few administrators are going to wait for 100% of their potential communication partners to support IPv6 before turning IPv4 off on their own side. As of today IPv4 is not supported by 100% of all deployed systems, and there are still lots of administrators choosing to make IPv4-only deployments. By whatever reasoning these administrators are using to choose IPv4-only today, they could choose IPv6-only once there are few enough IPv4-only systems left. IPv4 is not going to be dead by the end of the decade, but it may very well have been overtaken by IPv6.
    – kasperd
    Apr 17, 2016 at 17:00
  • +1 from me for some data.
    – MadHatter
    Apr 18, 2016 at 6:54
  • 1
    @Sebb The only IP address I see any need to remember is ::1. And no, IPv4 is not easier. IPv4 is slightly more complicated than IPv6. But what really makes things complicated is NAT, tunnels, and all the other workarounds caused by the futile attempt at keeping IPv4 alive forever. One very useful feature of IPv6 is that link-local addresses will work as soon as you have brought the interface up even if your routing and/or addressing is totally messed up.
    – kasperd
    Apr 18, 2016 at 13:40
  • 2
    @Sebb No problem. I start with ping6 -nc2 ff02::1%eth0, then I can see what the link-local address of the router is. Plus this works even if the router has not configured its network stack yet, which may happen if it is waiting "forever" for the ISP to send the configuration data.
    – kasperd
    Apr 18, 2016 at 14:03
  • 1
    Out of the list of major providers that I checked five years ago, only registrar.com has since added iPv6 addresses for their MX.
    – Law29
    Mar 27, 2021 at 13:50

The answer depends on your success criteria. But most likely will be no.

If you are running a business where any undelivered mail means a measurable cost. Then the answer is no, IPv6-only is not viable yet.

There are many providers including some large providers who are still running IPv4-only. The largest provider I know of with dual stack support is Gmail, but I frequently see delivery attempts over IPv6 being rejected by Gmail and then succeeding when retried over IPv4. So even for delivering mail to providers with dual stack, you are not going to experience reliable delivery.

If your primary goal isn't to be operating a reliable mail service for your users, but rather to make IPv4-only look less viable. And if you only need a high enough success rate to not have everybody laugh at you when you blame delivery failures on those running IPv4-only, then the answer is that IPv6-only could be almost viable today.

Hopefully in another two years most administrators will agree that IPv4-only is no longer viable, and dual stack will be a requirement.

  • 3
    I've also seen the behaviour where mail sent to Gmail first tries IPv6, but fails. In that case it was not having a reverse PTR set BUT when it was fixed the address kept changing due to RA/ND and Gmail kept greylisting (retry later) the delivery attempts each time the address changed
    – damolp
    Apr 18, 2016 at 5:00
  • 1
    +1 from me for the observation that v6 and v4 do not produce equivalent delivery results even for fully-dual-stack providers.
    – MadHatter
    Apr 18, 2016 at 6:55
  • I'd like to read some updated informations if available, becasue 2 years have passed since this reply, and it seems to me the situation is not much different than it was 2 years ago, but I may be wrong. I miself run several IPv4 only servers with no issue at all to date. Nov 7, 2019 at 18:21
  • Now 5 years have passed, has this answer aged well? Mar 26, 2021 at 22:00

Nope, not really. I think it's a very good idea to go ipv6-only and to boycott anything that doesn't do IPv6, but the Server-to-Server side of the mail server and the browser on my desktop are the two points where I have to make exceptions because there are still too many IPv4-only deployments out there that you simply can't avoid.

I have two servers. One of them is dual stacked, the other is IPv6 only. The latter is my main MTA and it uses the other (dual-stacked) server as a fallback MTA for any mail that doesn't go through directly. That other server is also my secondary MX which takes incoming IPv4-only mail. I even made one of my two DNS servers IPv6-only.

A few days ago, I made my IMAP and submission SMTP server IPv6-only. I thought this shouldn't be a big deal because all of my mail clients were already doing IPv6. Well, so I thought.

I was under the assumption that I could simply assume tacitly that any mail client would support IPv6 without ever verifying that assumption.

Actually, it turned out that out of the 4 mail clients I'm using, one lacks software support for IPv6. This went unnoticed because everything exact the mail client software itself was dual-stack. When I was done migrating the mail server, it suddenly turned out that one of the mail clients was dead.

Well, I'm now in contact with the developer to have the mail client fixed.

So the short answer is: Unless your MTA is limited to a special use case where IPv6-only is an option, you can't make your MTA ipv6 only in 2023.

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