Hi, is there any document that contrasts RFC 3974 with the real world? I try to create a set of MX RRs which includes an IPv6 MX host, does not negatively impact reachability from v4-only hosts (or v6-enabled hosts without v6 connectivity), prefers v6 over v4 when possible, and does not waste any IPv4 addresses. This is what I've come up with so far: deneb.enyo.de. 172800 IN MX 9 v6.mail.enyo.de. deneb.enyo.de. 172800 IN MX 10 v4.mail.enyo.de. v6.mail.enyo.de. 172800 IN A 212.9.189.167 v6.mail.enyo.de. 172800 IN AAAA 2001:14b0:202:1::a7 v4.mail.enyo.de. 172800 IN A 212.9.189.167 The A RR of v6.mail is necessary because some broken anti-spam checks reject mail from deneb.enyo.de if they cannot find an A record for the primary MX. The v4.mail MX host is needed because some MTAs suppress A record lookup if they discover an AAAA record (even if there's no v6 connectivity available). I wouldn't be too surprised when this setup leads to bouncing mail, too. 8-> Florian