Postfix (which Plesk uses for its mail service) does not support SNI (many certificates on a single service). So you would realistically only be able to protect the server service domains, not customer domains.
First, you’ll want to secure your mail server with a Let’s Encrypt certificate for vps.dutchconnex.com.
Following https://support.plesk.com/hc/en-us/articles/115000179934-How-to-secure-mail-server-with-Let-s-Encrypt-certificate (which I suppose you’ve already read), you’ll want to add Let’s Encrypt certificate for vps.dutchconnex.com into the server pool.
Then, change each domain’s MX records to the server’s hostname (vps.dutchconnex.com), rather than e.g. mail.dutchinnovisiongroup.com.