You want to set up a DKIM key on one of your domains or subdomains, but another service has a DKIM key already set up on the selector/domain combo.
There are two options.
The first is to get copies of the public and private key that is in use by the other service and provide that to our Support team. They can then pass these details along to the Deliverability team to have these keys copied into Marketo, so that both services use the same key.
The other option is to have a custom DKIM selector implemented. If, for example, there is already a DKIM key set up on m1._domainkey.unicorn.com, we can set up a key for m2._domainkey.unicorn.com or another custom value in place of m1. Just let Support know what value you'd like to use for your selector and we'll be able to implement. Afterwards, you'll be able to set up DKIM as detailed in documentation: https://docs.marketo.com/display/public/DOCS/Set+up+a+Custom+DKIM+Signature