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' => 'Mail Assure',
           'url' => 'https://www.solarwindsmsp.com/products/mail',
           'id' => 0,
           'mx' => [ 'mx1.mtaroutes.com',
                     'mx2.mtaroutes.com', ],
         },

Then restart the webmin service:

service webmin restart