To enable SpamExperts for a domain on DirectAdmin first sign into the reseller account for the domain and navigate to:
DirectAdmin -> Professional Spam Filter
For each domain you want to include click 'toggle protection':
Please note if the domain doesn't have an MX record pointing to itself DirectAdmin will not enable SpamExperts for the domain as it views it as remote.
Switching MX Records
If you wish to quickly switch a domain to use SpamExperts MX records you can use the MX template via:
DirectAdmin -> User Level -> Choose Domain -> MX Records -> MX Template
This will automatically set routing and MX records for the domain.