Growing amount of users like to access their web-applications through a DNS hostname. (i.e www.example.com/webapp > webapp.example.com. Most likely fired up by the nice service of Let’s encrypt which makes getting certificates for them so easy.
To achieve this nethserver has tools in place to host such a web-application on a nethserver-virtualhost and a netserver-reverseproxy to access them by a DNS record.
However, some web-applications require an apache virtualhost configuration file in httpd/conf.d/ .
(mattermost and SOGo among them)
This leads to a race-condition who becomes the default vhost as apache picks the aphetically first one it finds. Nethsesrvers defaultshosts are defined in conf.d/ virtualhosts.conf (80) and conf.d/nethserver.conf (443) in conjunction with conf.d/default-virtualhost.inc.
IMO the nethserver virtualhost implementation would be more ridged if the apache configuration files of nethserver.conf and virtualhosts.conf would be renamed to 000-nethserver.conf and 000-virtualhosts.conf .
In order they ‘always’ win the race-condition to defaulthost and we get the one which was intended.
Only if someone is stupid enough to name their apache configuration 0000xxxx.conf this would fail.
(Right?) 
Referring to