Change only 1 emails DNS to use another server but keep the other ones in my server [closed]

You can't. That's not how e-mail works.

An external e-mail server attempting to send e-mail to [email protected] will simply look up MX for example.com, and hand the e-mail to the server listed as MX for example.com.

E-mail to the entire example.com goes to whichever server is listed as MX for that domain, in your case Protonmail's Mail Server.

That mail server can, based on a lookup table of some sort, decide what to do with the e-mail:

• Deliver it locally to a mailbox.
• Forward it to another mailserver.
• Return a error message to the sender.
• Discard it silently.

That's basically it. If you want different e-mail accounts to be handled by different mail servers, you can't configure this in DNS. You'd have to configure Protonmail to forward mail for accounts they don't handle to your mail server. They probably doesn't offer this service.

Then your alternative is to configure your mail server to forward mail matching [email protected] to Protonmail's MX. This requires that their system accepts not being MX. They probably have automated checks that will tell you that you have an error in this scenario, but it may work.