If I add a default DKIM key (default._domainkey....) for a domain, could that interfere with email sent on behalf of that domain from 3rd part mailing services such as Google and Outlook?
To give context and clarify what I'm asking, I host multiple different websites in WHM/cPanel hosting. Many of these websites have contact forms that use PHPMailer on the local web server to send emails from the websites. I was going through all the accounts adding the default DKIM records to improve email deliverabity for these emails sent from the websites. Then I noticed this message in cPanel: "If you send email from another server, you must use this server’s DKIM key on the remote server." Many of these domains use a third party service for email. So that got me wondering if adding the default domain key could cause DKIM to fail when email is sent from those 3rd party services.
I realise that a different (from the default) DKIM key can optionally be added for such services (e.g. google._domainkey....) but if these 3rd party DKIM keys are not added to the DNS records for the domain, then what I'm wondering is if the default DKIM key comes into play for emails sent from those 3rd party services. And if it does come into play, then I guess it would cause a DKIM fail?