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Sign DKIM signature on outgoing emails for new mail domain

Don't know what DKIM is? Check our tutorial here: What is a DKIM DNS record.

Don't know where Amavisd config file is? check this tutorial: Locations of configuration and log files of major components.

iRedMail configures Amavisd to sign outgoing emails for the first mail domain you added during iRedMail installation. If you added new mail domain, you should update Amavisd config file to sign DKIM signature for it.

Let's say your first mail domain added during iRedMail installation is mydomain.com, and new mail domain is new_domain.com, please follow below steps to enable DKIM signing for outgoing emails of this domain.

Use existing DKIM key for new mail domain

if you already have a working DKIM and valid DKIM DNS record, it's ok to use this existing DKIM key to sign emails sent by other hosted mail domains. This way, you don't need to ask your customer who owns this new domain to add DKIM DNS record.

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Copy the dkim_key('mydomain.com........ line, changing to new hostname, but keep same cert path. You should now have 2 lines starting with dkim_key with differetent domains, but same file path.

Next, add one line in @dkim_signature_options_bysender_maps, after "mydomain.com" line like below:

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    "new_domain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Generate new DKIM key for new mail domain

If you or your customer prefer to use their own DKIM key, you can generate a new DKIM key and ask your customer to add DKIM DNS record. Refer to our tutorial to add DKIM DNS record.

amavisd-new genrsa /var/lib/dkim/new_domain.com.pem 1024
chown amavis:amavis /var/lib/dkim/new_domain.com.pem
chmod 0400 /var/lib/dkim/new_domain.com.pem

Note

# amavisd -c /etc/amavisd/amavisd.conf genrsa /var/lib/dkim/new_domain.com.pem

# amavisd -c /etc/amavisd/amavisd.conf genrsa /var/lib/dkim/new_domain.com.pem 2048

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");

Add one line after above line like below:

dkim_key('new_domain.com', "dkim", "/var/lib/dkim/new_domain.com.pem");
@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Add one line after "mydomain.com" line like below:

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    "new_domain.com"  => { d => "new_domain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Again, don't forget to add DKIM DNS record for this new domain. The value of DKIM record can be checked with command below:

# amavisd-new showkeys

After added DKIM DNS record, please verify it with command:

# amavisd-new testkeys

Note: DNS vendor usually cache DNS records for 2 hours, so if above command shows "invalid" instead of "pass", you should try again later.

Use one DKIM key for all mail domains

If you want to use one DKIM key for all mail domains, please follow steps below:

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");
@dkim_signature_options_bysender_maps = ({
    # catch-all (one dkim key for all domains)
    '.' => {d => 'mydomain.com',
            a => 'rsa-sha256',
            c => 'relaxed/simple',
            ttl => 30*24*3600 },
});

References