Sad Clouds via Postfix-users:
> On Thu, 19 Mar 2026 19:51:03 +0000
> Bluejay Adametz via Postfix-users <[email protected]> wrote:
>
> > > This works because the Postfix SMTP client randomizes the order of
> > > equal-preference IP addresses.
> >
> > Is the question about balancing between _your_ SMTP servers, or are you
> > looking at how the SMTP servers you're sending mail _to_ might be load
> > balanced?
> >
> > If the former, the question has a broader scope than what the postfix SMTP
> > client does. If the latter, then Wietse's answer covers it.
>
> I'm actually testing three scenarios:
>
> 1. Mail relaying through my cluster of servers by others.
> 2. Mail submission by MUAs.
> 3. IMAP access by MUAs.
>
> All of them should load balances correctly across replica SMTP and IMAP
> servers. DNS round-robin seems to be good enough for this and avoids
> complex load balancing proxies.
There is a problem with load balancers: if a domain has only one
load balancer IP address, then a sending MTA may not immediately
retry delivery after session failure.
With multiple IP addresses, a sending MTA will immediately try
another IP address, up to some implementation-defined limit.
A load balancer would make sense for the submission services (where
the client is not an MTA, and having a single IP address is normal).
Wietse
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