This question is a follow-up question to this one.
The problem is that my current E-Mail provider does not natively support DKIM-signing, but I need that DKIM feature to make certain API integrations regarding my emails (like sending emails from my payment service provider through my own domain).
The solution I'm starting to see, as long as my host does not DKIM-sign their outgoing emails, is to watch out for an email providing service that allows you to send emails in the name of your own domain including the DKIM-signature feature. Does such a service exist, via API call? I've no experience whatsoever in this area, and would be happy to know if such a setup potentially has a bad influence on delivery rates / other problems.
If the mail server signs using your domain and not theirs, and you post the matching public key in your DNS, it will all work fine. You do need to find a provider that lets you do that though, which they may do via CNAME records. Such services will often allow you to configure what the mail server does with your messages using additional params in an HTTP API, or via custom headers over SMTP, e.g. it might allow you to set a header like:
Sign-with-selector: myselector
Recommendations for services are off-limits for SO though.