Here is your LDIF with the appropriate changes:
dn: dc=mysubdomain,dc=mydomain,dc=com
changetype: add
objectClass: top
objectClass: domain
dc: mysubdomain
description: Some root stuff
dn: ou=mysubdomaincertificates,dc=mysubdomain,dc=mydomain,dc=com
changetype: add
objectClass: top
objectClass: organizationalUnit
ou: mysubdomaincertificates
description: Provide some descriptive text here.
dn: Mail=test@mysubdomain.mydomain.com,ou=mysubdomaincertificates,dc=mysubdomain,dc=mydomain,dc=com
changetype: add
objectClass: top
objectClass: person
objectClass: inetOrgPerson
cn: Test Test
sn: Test
Mail: test@mysubdomain.mydomain.com
userSMIMECertificate: file:///path-cert-file