On 2023-12-12 15:51:58 +0100, Wietse Venema via Postfix-users wrote:
Peter Wienemann via Postfix-users:
Dear Postfix experts,
checking the documentation for the relayhost parameter [0] I find no
indication how Postfix behaves in case of multiple relay hosts with
multiple DNS entries. Let us assume the following setting:
for each destination d in relayhost:
try to deliver the remaining recipients to destination d
if no recipients remain, stop.
defer any remaining recipients
Perhaps surprisingly, Postfix delivers mail to "destination d" for
all supported forms of "d", whether that is a domain name, [hostname],
[ipaddress], and with or without an explicit :port.
Thanks Wietse! Your pseudo-code clarifies the approach chosen by
Postfix. What still remains unclear to me is the order in which
destinations are tried. Let us again consider the following example:
relayhost = mail1.example.com, mail2.example.com
I think RFC 5321 requires that one starts with MX records before falling
back to A or AAAA records (unless [] notation is used). So my guess is
that the trial sequence starts in the following way:
1. Try all MX records for mail1.example.com.
2. What comes next? Try all MX records for mail2.example.com? Or try A
records for mail1.example.com? Or try AAAA records for
mail1.example.com? Or is some randomness involved?
Best regards,
Peter
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