Virtualmin → <domain> → Server Configuration → E-Mail Settings → Cloud mail filtering provider

Edit feature-mail.pl:

vim /usr/share/webmin/virtual-server/feature-mail.pl

For a new provider, add something like this under list_cloud_mail_providers (near the bottom):

         { 'name' => 'MailCleaner',
           'url' => 'https://www.sonoracomm.com',
           'id' => 0,
           'mx' => [ 'mc.virtualarchitects.com',
                     'mc2.virtualarchitects.com', ],
         },

Then restart the webmin service:

service webmin restart