I am running my mail server on a Ubuntu 14.04 box without any issue. To check emails I have installed roundcube using the deb packages with aptitude. The program was installed to /usr/share/roundcube and set up a conf in conf-available this way:
# Those aliases do not work properly with several hosts on your apache server
# Uncomment them to use it or adapt them to your configuration
Alias /roundcube/program/js/tiny_mce/ /usr/share/tinymce/www/
Alias /roundcube /var/lib/roundcube
# Access to tinymce files
<Directory "/usr/share/tinymce/www/">
Options Indexes MultiViews FollowSymLinks
AllowOverride None
<IfVersion >= 2.3>
Require all granted
</IfVersion>
<IfVersion < 2.3>
Order allow,deny
Allow from all
</IfVersion>
</Directory>
<Directory /var/lib/roundcube/>
Options +FollowSymLinks
# This is needed to parse /var/lib/roundcube/.htaccess. See its
# content before setting AllowOverride to None.
AllowOverride All
<IfVersion >= 2.3>
Require all granted
</IfVersion>
<IfVersion < 2.3>
Order allow,deny
Allow from all
</IfVersion>
</Directory>
Now on every domain hosted on this server, if I add /roundcube at the end of the url, I see the roundcube login page. As far as I know this is set by the first two lines in the conf file. I am not so strong in Apache configuration to see what should I do after commenting them to set up roundcube to be reachable only with a domain like webmail.my_mail_server.com This domain resolves correctly to my mail server. The issue is only on the apache side. Thanks for the help!