IPv6 in the SMTP Outgoing Service

Advanced Configuration of Axigen

 In the default configuration, Axigen does not use the IPv6 network when sending outgoing messages — this behavior can be changed by using the CLI interface.

There is a CLI configuration option in the SMTP outgoing context that controls if Axigen uses IPv6 when sending outgoing messages.

  • delivery via MX happens over IPv6 (query AAAA) only if enabled in config
  • delivery via smart host IP address happens over IPv6 irrespective of config (because sysadmin has written an IPv6 address in the smart host config)
  • delivery via smart host name happens over IPv6 (query AAAA) only if enabled in config

SMTP outgoing delivery mechanism

 Because Axigen's configuration capabilities are not sophisticated enough to allow the provisions described in RFC 6724 (e.g. handling broken IPv6) Axigen prefers the IPv4 network. That means that in a dual stack environment where an MX points to equal priority A and AAAA records Axigen will prefer delivering over the IPv4 network.

Axigen's outgoing delivery mechanism is described below:

  1. if smart host is used and the smart host host is in IP format (e.g. [2a02:XXXX:YYYY::226:bbff:fe1a:757e])
    1. delivery is attempted to the specified IP address (be it IPv4 or IPv6)
  2. if smart host is used and the smart host is a name (e.g smarthost.axigen.net )
    1. MX records are retrieved for the smart host name
      1. if no MX records are found
        1. A records are retrieved and added with increasing priority (first added A entry has priority 1, the second 2 and so on)
        2. if IPv6 is enabled for SMTP outgoing AAAA records are retrieved and added with increased priority (following the A records above)
      2. if MX records are found
        1. if the found MX record is of type A (IPv4) it is added as relay
        2. if the found MX record is of type AAAA (IPv6) and IPv6 is enabled for SMTP outgoing it is added as relay
  3. if smart host is not used normal MX delivery takes place