Ubuntu – How to get Postfix to send/forward/relay to a sub-domain located on another server

emailpostfixsmtpsubdomainUbuntu

I have a quick question. How do I setup postfix to send an email to another server (Exchange Server) when sending to an email address that has a sub-domain of our main server. For example, say our main server is mail.example.com and we have a Exchange server setup to receive emails from exchange.example.com. We have the MX records setup in our DNS and it receives correctly if we send from a GMail account. However, when we try to send an email from a @example.com account we get the following error:

Host or domain name not found. Name service error for name=exchange.example.com type=A: Host not found

I believe Postfix checks for local mailboxes first and if its setup with the domain it delivers to the local account, but in this case the sub-domain accounts are located in another server. Anyone have any thoughts on what I need to do within Postfix so it doesn't look locally for the exchange.example.com mailboxes?

I found relay_domains directive within Postfix but that doesn't seem to fix it when I add the sub-domain.

Thanks for your help.

Best Answer

It seems that you're trying to set up an SMTP gateway, and do not want to expose your Exchange Server to the "outside" network, right? Because usually I'd just set up the appropiate MX records for the domain.

Postfix, by design, won't accept mails for foreign domains, but postfix wouldn't be postfix if it wouldn't be possible. It just requires you to do a bit more work, for example, telling postfix which email-adresses are valid (it can't determine this by itself for a subdomain, naturally). It's well explained over here: http://www.knowplace.org/pages/howtos/smtp_gateway_for_multiple_domains_with_postfix.php

Related Solutions

Linux – How to correct Postfix’ ‘Relay Access Denied’

TLS just enables encryption on the smtp session and doesn't directly affect whether or not Postfix will be allowed to relay a message.

The relaying denied message occurs because the smtpd_recipient_restrictions rules was not matched. One of those conditions must be fulfilled to allow the message to go through:

smtpd_recipient_restrictions =
    permit_sasl_authenticated
    check_recipient_access hash:/etc/postfix/filtered_domains
    permit_mynetworks
    reject_unauth_destination

To explain those rules:

permit_sasl_authenticated

permits authenticated senders through SASL. This will be necessary to authenticate users outside of your network which are normally blocked.

check_recipient_access

This will cause postfix to look in /etc/postfix/filtered_domains for rules based on the recipient address. (Judging by the file name on the file name, it is probably just blocking specific domains... Check to see if gmail.com is listed in there?)

permit_mynetworks

This will permit hosts by IP address that match IP ranges specified in $mynetworks. In the main.cf you posted, $mynetworks was set to 127.0.0.1, so it will only relay emails generated by the server itself.

Based on that configuration, your mail client will need to use SMTP Authentication before being allowed to relay messages. I'm not sure what database SASL is using. That is specified in /usr/lib/sasl2/smtpd.conf Presumably it also uses the same database as your virtual mailboxes, so you should be able enable SMTP authentication in your mail client and be all set.

Postfix: Recipient address rejected: User unknown in local recipient table

I know this question is a little old, so I'm assuming it's been answered satisfactorily already.

I just had this same issue, and it took me a while to figure out what was going on. I think my situation was the same as the original question.

Postfix should relay all mail to other servers on the internet, it does not actually receive mail for any domains. So any mail sent to example.com should be forwarded to the mail server for example.com. The solution, as explained b techieb0y, is to remove $mydomain from the line:

mydestination = $mydomain, localhost.$mydomain, localhost

This line tells postfix that any messages sent to $mydomain are to be received and stored on this server. That's not what I want, I want those messages to be sent to the actual mailserver for example.com. Once I realised this, and removed example.com, mail worked as I expected. I'm posting this on the off chance that this explanation helps somebody else who stumbles upon this question in the future.

Best Answer

Related Solutions

Linux – How to correct Postfix’ ‘Relay Access Denied’

Postfix: Recipient address rejected: User unknown in local recipient table

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