Re: [exim] Using DA POP3 together with Exchange

Pierre Munnichs <p.munnichs@???> wrote:

> I'm strugling with this problem for a few weeks now. Hope somebody can
> help me with this.

> I've got a webserver with DirectAdmin (DA). On this webserver i'm
> hosting a domain, for example mydomain.com.

> I've got 2 POP3 e-mail accounts on this domain:
> - email1@???
> - email2@???

> Then i also have an external Exchange 2010 server. The Exchange server is
> taking care for another 2 e-mail adressen on the same domain.
> - email3@???
> - email4@???

> Is it posible to configure exim that when an e-mail message arrives it
> checkes if that user exists, otherwise reject the message so it will go to
> the next adres in de MX records list?

If you reject a mail (with an 5xx response), it is gone, the delivering
MTA _will_ bounce it.

If you defer a mail (with an 4xx response) there is no guarantee the
delivering MTA will try the next MX or even when it will retry the
transmission. With some bad luck, it could always try the first MX and
then bounce the mail after some hours/days.

Some services like Yahoo are know to throw away mails which have been in
their queue for only 1 hour (!!), so better get the mail out of their
queue as fast as possible.

> For example:
> mail send to: email1@??? -> deliver to POP3 box on DA webserver.
> mail send to: email3@??? -> doesn't exist on DA server,reject ->
> forward to next MX records what will be the Exchange server.

You probably want your Exim4 MX to receive all mail for mydomain.com
(thus being the only MX) and just forward any non-local users to your
Exchange server.

This can be achieved by two routers, the first one to check local users
(normally already existing) and a second one after that, which routes
all other mails to your Exchange server. Maybe with a little recipient
lookup to reject mails to doesnotexist@??? instead of blindly
forwarding them along and causing late bounces which will result in
DNSBL entries.

Grüße,
Sven.

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