Just put it in VirtualHost section "VpopMailVirtualHost On" and UseCanonicalName Off
example
---- snip ---
UseCanonicalName Off
<VirtualHost *>
VpopMailVirtualHost On
ErrorLog /var/log/httpd/error_log
CustomLog /var/log/httpd/access_log combined
ServerAdmin webmaster@dummy-host.example.com
</VirtualHost>
---- snip ---