DKIM keys for specific domains + multiple domains

[John Luk]

hi, in our amavis we need to sign DKIM for a few domains using
corresponding keys, AND also need to use one key to sign for some other
domains.  As I can collect, the relevant conf could look like:

dkim_key('domain1.com', "dkim",
"/var/lib/dkim/domain1.com.pem");dkim_key('domain2.com', "dkim",
"/var/lib/dkim/domain2.com.pem");
dkim_key('others.com', "dkim", "/var/lib/dkim/others.com.pem");
@dkim_signature_options_bysender_maps = ( {
    "."  => { d => "others.com", a => 'rsa-sha256', ttl => 10*24*3600 },
});

In my understanding the above would:

* sign domain1.com using that domain1.com key
* sign domain2.com using that domain2.com key

* sign domainX.com using that others.com key
* sign domainY.com using that others.com key

Is that true?

I can't find doc that definitely support the above, can someone point
me to the right link?

https://www.ijs.si/software/amavisd/amavisd-new-docs.html appears to
suggest that although I'm not quite sure.

How to make sure others.com key is NOT used to sign for domain
domain1.com ? As that will create big chaos for the existing domain.

Thanks!
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