http://www.virtualmin.com/documentation/id,webmail_and_virtualmin/#webmail_redirects

Modify the desired redirects, usually webmail:

System Settings → Server Templates → Default Settings → Apache website → URL for webmail redirect

Then, for the specific domain or all domains, recreate the redirects:

virtualmin modify-web --domain yourdomain.tld --no-webmail
virtualmin modify-web --domain yourdomain.tld --webmail

virtualmin modify-web --all-domains --no-webmail
virtualmin modify-web --all-domains --webmail