From IPv6 to IPv4

One can set up a single dual-stack mail server to act as a so called ‘smart host’ (smart mail relay) for IPv6-only hosts. A sendmail configuration file ‘sendmail.cf’ is typically generated from an m4 macro file. One should add the following lines to this m4 macro file:

DSmail64.mydomain.tld

Here ‘mail64.mydomain.tld’ is the domain name of the dual-stack mail server that has been set up.

From IPv4 to IPv6

Two methods can be used to relay email through a dual-stack sendmail server to an IPv6-only mail server. The first method is by defining IPv6-only relays in the configuration file of the dual-stack sendmail. An example of this is to set the so-called ‘LUSER_RELAY’ in the dual-stack sendmail to the IPv6-only mail server in such a way that all email destined for email accounts not known at the dual-stack server are relayed to the IPv6-only mail server. This configuration for the dual-stack sendmail can be generated by adding a line lithe the following to the m4 macro file it is generated from:

define(`LUSER_RELAY', `relay:mail6.mydomain.tld')

The second method to relay email through a dual-stack sendmail server to an IPv6-only mail server(from IPv4-only hosts) is by setting up the MX records for the domain in question appropriately. Let us take as example the domain ‘mydomain.tld’ and assume that an (optional) IPv4-only mail serve ‘mail4.mydomain.tld’, a dual-stack mailserver ‘mail64.mydomain.tld’ and an IPv6-only mail server‘mail6.mydomain.tld’ have been set up for this domain.

An appropriate DNS configuration should then list MX records in the following order of priority:

In other words, the IPv6-only mail server(s) should have lowest priority code (meaning highest priority mail server), followed by the dual-stack mail server(s) and finally the IPv4-only mail server(s), if any, should be given the highest code(s) (and thus the lowest priority). Of course, mail6.mydomain.tld should have an AAAA DNS record, mail64.mydomain.tld should have both an AAAA and an A DNS record, while mail4.mydomain.tld would only have an A record in DNS.

IPv4 clients will then send email to (one of) the IPv4 capable mail servers, which will relay it to (on of the) IPv6-only server(s). In the example above, an IPv4 client will send its mail to the first mail server on the list that has an IPv4 address: the dual-stack server mail64.mydomain.tld. Only if mail64.mydomain.tld is unreachable or down will it (try to) send the email to mail4.mydomain.tld. After mail64.mydomain.tld has received the email it will relay it to the higher priority mail server mail6.mydomain.tld, which is IPv6-only.

IPv6 capable clients will try to send their email for domain mydomain.tld directly to the IPv6-only mail server mail6.mydomain.tld. If this is (temporarily) unreachable those clients will instead use the dual-stack server mail64.mydomain.tld.

Note that most sites will want to have a backup mail server for both IPv6 and IPv4. This means that at least two of the mail servers need to be IPv6 capable and two of them need to be IPv4 capable as in the example above.