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Paweł Gościcki
Asked: 2010-04-01 08:22:10 +0800 CST

SMTP for multiple domains on virtual interfaces

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The setup is like this (Ubuntu 9.10):

eth0: 1.1.1.1 name.isp.com

eth0:0 2.2.2.2 example2.com

eth0:1 3.3.3.3 example3.com

example2.com and example3.com are web apps which need to send emails to their users.

2.2.2.2 points to example2.com and vice-versa (A/PTR). MX -> Google. Google handles all incoming mail.

3.3.3.3 points to example3.com and vice-versa (A/PTR). MX -> Google. Google handles all incoming mail.

Requirements:

  1. Local delivery must be disabled (must deliver to MX specified server), so that the following works (note that there is no local user bob on the machine, but there is an existing bob email user):

    echo "Test" | mail -s "Test 6" [email protected]

  2. I need to be able to specify from which IP/domain name the email is delivered when sending an email.

I fought with sendmail. With not much luck.

Here's some debug info:

sendmail -d0.12 -bt < /dev/null

Canonical name: name.isp.com
UUCP nodename: host
    a.k.a.: example2.com
    a.k.a.: example3.com
    ...

Sendmail always uses canonical name (taken from eth0). I've found no way for it to select one of the UUCP codenames. It uses it for sending email:

echo -e "To: [email protected]\nSubject: Test\nTest\n" | sendmail -bm -t -v

[email protected]... Connecting to [127.0.0.1] via relay...
220 name.isp.com ESMTP Sendmail 8.14.3/8.14.3/Debian-9ubuntu1; Wed, 31 Mar 2010 16:33:55 +0200; (No UCE/UBE) logging access from: localhost(OK)-localhost [127.0.0.1]
>>> EHLO name.isp.com

I'm ok with other SMTP solutions. I've looked briefly at nbsmtp, msmtp and nullmailer but I'm not sure they can deal with disabling local delivery and selecting different domains when sending emails.

I also know about spoofing sender field by using mail -a "From: <[email protected]>" but it seems to be a half-solution (mails are still sent from isp.com domain instead of proper example2.com, so PTR records are unused and there's more risk of being flagged as spam/spammer).

linux email smtp sendmail email-server

3 Answers

  1. I do not know about sendmail, but with exim you specify routers and transports. Routers take the email from/to addresses and decide which transport to use. For (1), Just create smtp transports and no local transports. For (2), you can specify an interface option when you create a transport. A config file would be something like this (not a complete config file and not tested):

    begin routers

example2_route:
 driver = dnslookup
 condition = ${eq{$sender_address_domain}{example2.com}} # 'from' domain is example2.com
 transport = example2_smtp

example3_route:
 driver = dnslookup
 condition = ${eq{$sender_address_domain}{example3.com}}
 transport = example3_smtp

begin transports

example2_smtp:
 driver = smtp
 interface = 2.2.2.2

example3_smtp:
 driver = smtp
 interface = 3.3.3.3
    • 1

  2. Did you try using the mailertable? Mapping

    do.main    esmtp:do.main

    should work and make sendmail use the MX-record (untested).

    • 0

  3. I have setup multiple postfix instances in the manner you describe to service outgoing mail for two namespaces. It is a fairly involved procedure (mine more so since I am also DKIMProxy signing) but a good starting point is: http://advosys.ca/papers/email/58-postfix-instance.html

    I documented the whole thing (on RHEL 5.6) so I can provide more detail, so let me know if you're going this route - I can provide additional info.

    The Exim approach posted by Dan looks cleaner - I stuck with postfix/DKIMProxy since I've used this approach in the past.

    Cheers

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