I read him/her as referring to the mail server lacking an IPv6 reverse DNS address. This is a fairly common oversight, since IPv6 connectivity is getting more c

I read him/her as referring to the mail server lacking an IPv6 reverse DNS address. This is a fairly common oversight, since IPv6 connectivity is getting more common, but many providers' control panels allow only easy self-setting of the IPv4 RDNS. Many mailservers are set up to reject mail from senders who lack an RDNS (and/or have one that doesn't forward-resolve back to the original IP), so in that situation, if you connect over IPv6 you may get rejected. The solution is to either disable IPv6 (less preferred option), or set a reverse DNS name for your IPv6 address (more preferred).