Jaroslaw Rafa:
> Dnia 14.10.2020 o godz. 13:38:12 Wietse Venema pisze:
> > Here's some email basics.
> >
> > 1) You arrange for an MX and/or A record in your DNS zone. You edit
> > the zone file yourself, or you use some provider's application to
> > edit their zone file.
> >
> > example.com 10 IN MX mail.example.com.
> > mail.example.com. IN A 10.0.0.2
> >
> > 2) SMTP uses destination port 25 for MTA-to-MTA traffic, therefore
> > the port information is not in the DNS.
> >
> > 3) Some remote MTA looks up your MX and/or A record and connects to
> > your Postfix servers on port 25.
>
> I think there's one important thing to add.
>
> If you have a setup as above, then for mail addressed to "[email protected]",
> the remote MTA checks the MX record for example.com domain, finds out that
> it points to mail.example.com, checks the A record for mail.example.com and
> connects to the IP address found.
>
> However, if the sender addresses the email to "[email protected]", the A
> record for mail.example.com is sufficient to have mail delivered (assuming
> your Postfix is configured to honor both example.com and mail.example.com
> names as "mydestination"). The MX record for example.com domain is not
> involved in the process, as the domain in the e-mail address is
> mail.example.com and not example.com.
>
> If there is another server within the example.com domain and it has it's own
> independent Postfix instance, if you add A record for that server to the
> zone file, you can send mail directly to it:
>
> othermail.example.com. IN A 10.0.0.4
>
> Then messages addressed to "[email protected]" will go to that
> other server, while messages to "[email protected]" will still go to
> mail.example.com.
>
> You should not try to add another MX record for example.com domain pointing
> to othermail.example.com, because if you do this, and email service on both
> servers is not synchronized (which is not quite easy to do), the remote MTA
> sending mail to "[email protected]" will connect randomly to mail.example.com
> or othermail.example.com, so the message will end up at random on one or the
> other server (but never on both).
Except when the MX records have different preferences:
example.com 10 IN MX mail.example.com.
example.com 20 IN MX othermail.example.com.
mail.example.com. IN A 10.0.0.2
othermail.example.com. IN A 10.0.0.4
Then, Postfix on othermail.example.com will forward [email protected]
to the primary MX host mail.example.com (assuming that mydestination
is configured correctly, i.e. it does not contain example.com).
Wietse
