[Postfixbuch-users] Zwei MXer und (geplanter) Ausfall eines Servers

[Mathias Jeschke]

Hi Joe,

Am 01.07.13 10:30, schrieb Joe Schönberg:

> Es war mir jedenfalls bisher nicht klar, daß/ob ein MTA-Client
> bei zwei gleich gewichteten MXen bei Mißerfolg mit einem MX sofort 
> den anderen nimmt. Peer sieht es jedenfalls so.

Das sieht nicht nur Peer so, sondern auch die IETF (die für SMTP
verantwortlich ist) - siehe RFC 2821:

> 5. Address Resolution and Mail Handling
> 
> [...]
> 
> When the lookup succeeds, the mapping can result in a list of
> alternative delivery addresses rather than a single address, because
> of multiple MX records, multihoming, or both.  To provide reliable
> mail transmission, the SMTP client MUST be able to try (and retry)
> each of the relevant addresses in this list in order, until a
> delivery attempt succeeds.  However, there MAY also be a configurable
> limit on the number of alternate addresses that can be tried.  In any
> case, the SMTP client SHOULD try at least two addresses.
> 
> Two types of information is used to rank the host addresses: multiple
> MX records, and multihomed hosts.
> 
> Multiple MX records contain a preference indication that MUST be used
> in sorting (see below).  Lower numbers are more preferred than higher
> ones.  If there are multiple destinations with the same preference
> and there is no clear reason to favor one (e.g., by recognition of an
> easily-reached address), then the sender-SMTP MUST randomize them to
> spread the load across multiple mail exchangers for a specific
> organization.
> 
> [...]

Just my 0,02 €

Mathias.