I created the srv record saying to look at mail.soko.nl

I created cname autodiscover.3r.nl but outlook then says certificate isn't allright. looking for cert for autodiscover.3r.nl but finding mail.soko.nl

Hello Richard. Unfortunately no, your certificate for your primary domain will not work on customer domains. Most email clients and mobile devices default to customer-domain.com, mail.customer-domain.com, or autodiscover.customer-domain.com when probing for account set up details, and so it is best to have these hostnames set up to terminate on the SmarterMail server, then implement an SSL certificate that can cover all of these required hostnames in IIS. A good way to go about that is using Lets Encrypt as you'll see here: 

Try changing the autodiscover entry on the customer domain from an A record (your server IP address) to a CNAME record (your certificate host name).   Of course, you still need an A record that maps your certificate host name to an IP address, but I assume that this is already in place.   I think this will allow the existing certificate to validate successfully.