Jason Long:
> Thank you, but I never got my answer.1- Postfix or Dovecot has any
> option about changing default record that a mail server using?2-
> Could A record offer MX record in my goal?
An SMTP server cannnot tell remote systems what TCP port they should connect
to, or what DNS record they should ask for.
Wietse
> On Wed, Oct 14, 2020 at 10:09 PM, Wietse Venema<[email protected]>
> wrote: 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
>
>
