So I’ve had a go at adding the public key to DNS. Now what do I do with the private key ? The article says “save the private key to your SMTP server”. I gather this is something to do with me email. Are we talking about my personal email address or the one linked to my domain ? And (in basic terms please) where do I need to be looking and what action do I need to take?
Whatever key you are using is unlikely to be correct. Unless you are the system administrator of a mailserver, you should not have a private DKIM key. You need to use the DKIM public key provided to you by the operator of your domain email.
DKIM signing, which is what the keys are used for, is a domain level email function. Unless your personal email is also in a domain that you control, it will have no involvement in your DKIM and DMARC configuration.
The domain provider is IONOS. This is their guidance page - What is a DKIM Record and how to create it - IONOS. I don’t understand why they don’t provide the DKIM keys themselves as they provide the email address that I’m trying to authenticate, but there you go.
I’ve generated keys manually in the way that they suggest and added the public key to DNS.
You appear to working from the section labeled:
Generate DKIM keys manually
You need to use the section above it:
Request DKIM keys from an email provider