Wietse:
> Yes, if the sender is Postfix. Postfix will randomly select from
> equal-preference IP addresses and IP protocols. This is intentional,
> so that mail does not get stuck when one path is broken.
See also my second response with pointers to relevant Postfix
configuration parameters that control the order of IP addresses and
how many IP addresses Postfix will try before giving up.
> Other MTAs may use a different approach, For example, they may try
> only one IP address per MX record, or they may try IPv4 only after
> they fail to deliver mail over IPv6.
>
>
> Hi Wietse,
Bauer, Stefan (IZLBW Extern):
> is this behavior not described in an RFC? Or is it up to the MTA
> to deal with the dns answers? Do you know how Microsoft
> Exchange-Mailservers are behaving in the above setup?
If one MX record expands into multiple IP addresses, then RFC 5321
assumes that those addresses belong to the same host, and says that
the SMTP client must try those addresses in the order as presented.
That is the dumbest idea in the RFC. Postfix takes a different
approach so that mail doesn't get stuck trying the same IP addresses
in the same order over and over and over again.
I have no idea how non-Postfix SMTP clients implement MX/A lookup.
Wietse
