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[xy16644]

I'm hoping someone can help me or explain to me when IPv6 with SMTP is used over IPv4. Let me explain. I have setup IPv6 with my email server. It has an IPv4 address as well. When I send an email FROM Gmail to my email server it all gets sent using IPv6 so no issues here (I check this in the email headers and Exchange logs).

What I am battling with is when I send email from my email server to Gmail it seems to use IPv4 90% of the time. I can't seem to understand why IPv6 is so rarely used when sending TO Gmail? Why does IPv4 get used most of the time while IPv6 gets used very rarely when Gmail supports IPv6 email? I just can't seem to pinpoint a pattern here!

Yes, I have a PTR record setup for IPv6.

Any ideas? Also, is there another way to test IPv6 email besides using Gmail?

Thanks.

[passport123]

What MTA do you use on your email server?  Does that MTA have any configuration parameters that specify a preference for IPv4 over IPv6?

For example:
http://www.postfix.org/postconf.5.html#smtp_address_preference


Also, are the MX addresses of the destination of equal priority for the IPv4 and IPv6 addresses.

[xy16644]

What MTA do you use on your email server?  Does that MTA have any configuration parameters that specify a preference for IPv4 over IPv6?
For example:
http://www.postfix.org/postconf.5.html#smtp_address_preference

I'm using the Exchange 2016 Edge server as my MTA. I thought IPv6 was always prefered over IPv4? Correct me if I am wrong  I have had a look at the send connector but couldn't find anything wrong.

Also, are the MX addresses of the destination of equal priority for the IPv4 and IPv6 addresses.

I had a look at gmail.com's MX records and they are as follows:

gmail.com   IN   MX   
preference:   5
exchange:   gmail-smtp-in.l.google.com

gmail-smtp-in.l.google.com   IN   A   108.177.9.26
gmail-smtp-in.l.google.com   IN   A   108.177.9.27
gmail-smtp-in.l.google.com   IN   AAAA    2607:f8b0:4003:c13::1b

gmail.com   IN   MX   
preference:   10
exchange:   alt1.gmail-smtp-in.l.google.com

alt1.gmail-smtp-in.l.google.com   IN   A   173.194.219.26
alt1.gmail-smtp-in.l.google.com   IN   A   173.194.219.27
alt1.gmail-smtp-in.l.google.com   IN   AAAA    2607:f8b0:4002:c03::1a

gmail.com   IN   MX   
preference:   20
exchange:   alt2.gmail-smtp-in.l.google.com

alt2.gmail-smtp-in.l.google.com   IN   A   209.85.232.26
alt2.gmail-smtp-in.l.google.com   IN   A   209.85.232.27
alt2.gmail-smtp-in.l.google.com   IN   AAAA    2607:f8b0:400d:c0d::1a

gmail.com   IN   MX   
preference:   30
exchange:   alt3.gmail-smtp-in.l.google.com

alt3.gmail-smtp-in.l.google.com   IN   A   173.194.214.26
alt3.gmail-smtp-in.l.google.com   IN   A   173.194.214.27
alt3.gmail-smtp-in.l.google.com   IN   AAAA    2607:f8b0:400c:c0b::1b
   
gmail.com   IN   MX   
preference:   40
exchange:   alt4.gmail-smtp-in.l.google.com

alt4.gmail-smtp-in.l.google.com   IN   A   64.233.186.26
alt4.gmail-smtp-in.l.google.com   IN   A   64.233.186.27
alt4.gmail-smtp-in.l.google.com   IN   AAAA    2800:3f0:4003:c00::1b

The strange thing is that in Gmail in the email header is says:

Received: from MAIL.domain.com (mail.domain.com. [213.xxx.xxx.xxx])
        by mx.google.com with ESMTPS id k84si25404388wmb.52.2016.05.03.00.41.51

I did a lookup of mx.google.com and it doesn't resolve to anything so I am stumped at this stage. Each MX record has A and AAAA records so I thought the AAAA (IPv6) MX record would have been used as a prioroty over the IPv4 one?

[passport123]

...
I'm using the Exchange 2016 Edge server as my MTA. I thought IPv6 was always prefered over IPv4?
...

The preference depends upon the decisions made by the MTA which, in part, may be determined by how it is configured.

It appears that you have working IPv6 at the server (you get inbound IPv6 mail), you can get IPv6 DNS results, and I don't see any preference for IPv4 over IPv6 in google's MX records. 

If it were my server, my next step in troubleshooting would be to assure the MTA is not configured to give IPv4 a preference over IPv6 for outbound email.  Maybe pose a question on the Exchange support forum?

[xy16644]

Ok, thanks for the help. I'll see what the Exchange forums say!

[passport123]

Also, keep in mind that the main goal of an MTA for outgoing mail is to deliver the mail. 

Whether IPv4 or IPv6 is used to deliver the mail is quite secondary to the goal of assuring the mail gets delivered.