If you are using EA DKIM/DomainKeys plugin to sign the DKIM/DomainKeys signature on your IIS SMTP or Exchange server for a domain, but you also want to sign the DKIM/DomainKeys on another MTA that is not using EA DKIM/DomainKeys for the same domain. The problem is EA DKIM/DomainKeys uses PCS #12 certificate to store the key-pair, and if your other MTA DKIM plugin doesn't support the PCS #12 certificate, then you cannot use the same key-pair on other MTA.


The common solution is selector:

For example, in your current Exchange DKIM, you used s1024 as the selector.
Then your public key is deployed to s1024._domainkey.yourdomain

When you send the email from your Exchange server, the email will be signed by EA DKIM plugin and the receiver will query the public key from s1024._domainkey.yourdomain to validate the DKIM signature.

In your other MTA, you can generate the key-pair by your MTA DKIM tool.
Then you can create another selector named: mta, then your public key based on this MTA will be deployed to mta._domainkey.yourdomain.

When you send the email from this MTA, the email will be signed by this MTA DKIM plugin and the receiver will query the public key from mta._domainkey.yourdomain to validate the DKIM signature.

You can see “selector” solved the different key-pair/certificates problem with the same domain on multiple servers.