Unlike Apache 1.3, Apache 2 requires more than one module for mod_proxy functionality. Apache 1.3 requires only mod_proxy for basic proxy support. Apache 2 requires mod_proxy in addition to protocol-specific modules depending on the type of proxying required:

LoadModule proxy_module            /path/to/mod_proxy.so
LoadModule proxy_http_module       /path/to/mod_proxy_http.so

ProxyRequests off

<Proxy *>
   Order deny,allow
   Allow from all
</Proxy>

<VirtualHost *:80>
   ServerName localhost
   UseCanonicalName Off
   ProxyPass /other/ http://otherserver:8888/
   <Location /other/>
      ProxyPassReverse /
   </Location>
</VirtualHost>

If you want to take your site down for maintenance, but leave your webmail portal active on Sundays, you could put this in either your <VirtualHost> block or in an .htaccess file for your site:

RewriteEngine on
RewriteCond %{TIME_WDAY}         0
RewriteRule !^/mail.*            https://path/to/your/maint/page.html

Some virtual mapping goodness:

<VirtualHost *>
  ServerName cprogrammer.org
  ServerAlias cprogrammer.org *.cprogrammer.org

  # See the docs for the %-specs; %1 maps to the first part of the
  # domain (a.b.c.d.e -> a, a.b.c -> a)
  VirtualDocumentRoot /users/%1/public_html
  VirtualScriptAlias /users/%1/public_html/cgi-bin/
</VirtualHost>