Forward email to other servers if the recipient doesn’t exist in alias map

emailpostfix

This is the first time I am starting to write a more complex postfix as I hate setting up mail settings without knowing what to do. But right now I am required to do.

Here the setting:

For internal reasons I am required to have my postfix relay set up like this:

If recipient of an email is doesn't exist in alias map but the recipient domain does, postfix will send it to real mail server.

Okay, this reads strange, but I am giving you an example.

MX record of example.com is set to ProxyHost but the real mailserver with all accounts is reachable under external.example.com

In mysql I have setup redirect@example.com should be forwarded to my@example.net. This is working right now.

But if I am sending a mail to whatever@example.com where whatever is NOT in the local mysql forwarding table but as you can see example.com is, I would love to forward the mail to the real mail server. This real server information was located in the domains table in the realmx column.

This does not look very hard on me, but I am not able to handle it. Right now all mails to whatever@example.com are just getting rejected.

Best Answer

That's a very standard and typical setup. Postfix runs on a server which is external facing and receives mail and passes it on to the server which has the mailbox:

world -> smtp.example.com -> imap.example.com

On your smtp server (which you call ProxyHost but I won't use this term as it's not really a proxy in the general usage of the term), you will use configurations as follow (you'll have to adjust to your exact scenario, these are general guidelines):

mydestination = localhost
transport_maps = hash:/etc/postfix/transport
relay_domains = example.com
relay_recipient_maps = hash:/etc/postfix/relay_recipients

/etc/postfix/relay_recipients will contain the list of valid recipients.

/etc/postfix/transport will list the destination for the final destination of the mail.

Full documentation and complete explanation of each configurations above can be found in the great Postfix manual. Start with the relay_domains documentation.

Related Solutions

Postfix change to address based on sending host

A not so elegant solution would be to use REDIRECT in header checks. It will send your email to desired address but it will not rewrite the TO header.

In main.cf uncomment or put

header_checks = regexp:/etc/postfix/header_checks

if it is not already.

In /etc/postfix/header_checks put something like:

/^Received:.*app1.example.com.*/ REDIRECT testing@example.com

If reverse DNS does not work correctly you better use the IP instead.

Don't forget to run postmap /etc/postfix/header_checks

How to route some email accounts of a domain with transport_maps to another MX if user doesn’t exist locally

Why doesn't the routing happen before the recipient existence check? Isn't there is way? When I route emails, I mean "The current Postfix doesn't handle anything and just forwards email to another server which is fully responsible for the config".

No, that isn't way postfix gonna works. Recipient existence checks and email routing was two different activities for postfix. Recipient existence checks was performed when accepting email. The process who performs it was smtpd. Email routing was performed when delivering email. The process who perform it was trivial-rewrite

In the comment, you mention that you use virtual delivery agent from postfix to delivering email. I'm gonna tell you that it possible to achieve no-redundant-information in postfix configuration. BUT you must change current postfix setup.

To achieve your goal, we must move all domains in virtual_mailbox_domain class to relay_domains class. Then using dovecot LDA to deliver email in the right mailbox. Relay domains class has special parameter called relay_recipient_maps to checks the recipient existence.

The good news is you can use relay_recipient_maps together with transport_maps. Why? Because relay_recipient_maps only used to check existence of user. The value/right hand side string in transport_maps was ignored when recipient existence check. Snippet from man postconf.

Optional lookup tables with all valid addresses in the domains that match $relay_domains. Specify @domain as a wild-card for domains that have no valid recipient list, and become a source of backscatter mail: Postfix accepts spam for non-existent recipients and then floods innocent people with undeliverable mail. Technically, tables listed with $relay_recipient_maps are used as lists: Postfix needs to know only if a lookup string is found or not, but it does not use the result from table lookup.

For the completeness, here the postfix configuration for this setup. Credits to sebokopter for the idea.

#main.cf
# rename virtual_mailbox_domains to relay_domains
relay_domains = mydomain.example.com
# put the parameter value of virtual_mailbox_maps AND transport_maps parameter
relay_recipient_maps = hash:/my/virtual_mailbox_maps, hash:/my/transport_maps

transport_maps = hash:/my/transport_maps
relay_transport = dovecot:

#master.cf
dovecot   unix  -       n       n       -       -       pipe
    flags=DRhu user=vmail:vmail argv=/usr/local/libexec/dovecot/dovecot-lda -f ${sender} -d ${recipient}

Best Answer

Related Solutions

Postfix change to address based on sending host

How to route some email accounts of a domain with transport_maps to another MX if user doesn’t exist locally

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