Pim Zandbergen:
> Wietse Venema wrote:
> > I know of no RFC that says only whitelisted clients can send email
> > over IPv6.
>
> Well, it's their policy. I can respect that, if their assumption that
> senders should fall back to IPv4 is valid.
This policy is mistaken for the following reasons.
First, they reject the mail deep in the SMTP conversation instead
of in the protocol handshake phase. This means that some clients
will never deliver mail at all. These clients try IP addresses until
an SMTP server responds, and will not try other MX hosts. This
behavior is observed with with qmail and Postfix <= 2.0. This
behavior is compliant with the SMTP RFCs.
Second, the SMTP RFCs says that a sender should try at least two IP
addresses. If the receiver announces multiple IPv6 addresses that
refuse email after the SMTP protocol handshake, then they will
experience email delays or they won't even receive mail at all from
non-whitelisted IPv6 systems, depending on the client's SMTP-over-IPv6
implementation.
Wietse
