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?
DKIM verifies the sender and the integrity of a message. In addition, it allows an email system to prove that spammers did not alter an incoming message while in transit. DKIM also verifies that the messages your domains receive come from the specified domain.
DKIM are important for ensuring that your domain is not delivered to your recipients spam folders or even outright rejected. Mostly hosting providers enables these by default for all cPanel accounts. Third-party DNS providers may require you to add these records manually.
So, answering your question is yes having wrong DKIM record will be a problem. You should add your 3rd party DKIM record in your cpanel.
However missing/ not having DKIM is different scenario as it shouldn't be a problem. Having the DKIM key on your messages is a 'advantage' as to indicate the spam filters and other mail servers that these messages are valid messages. But, not having the DKIM key would not be considered a 'Fail'.