Hi Michael
It works best if you have DNS services configured internally - ideally matching your external domain. Make sure IMAP is enabled for desired users (use WGM for this) and make sure ports 143 and 16080 are opened on your router and forwarding to your mail server's internal IP address. This is if you want to allow external access?
Web Services uses an asterisk for the default website. If you have only one site then you could leave it as it is. What I normally do is to edit it to read the fqdn of your domain with its associated IP address. Thereafter select the WebMail option in the Sites tab as well as WebDAV. Make sure the webdav module is ticked in the Modules section. That should be it? On the client launch your preferred web browser and in the url key in:
http://<server'sprivateipaddress>:16080/webmail
If your clients are using to server's DNS service to resolve DNS queries then you could use the servers FQDN instead of its IP address. The choice is yours.
If you do want external access then simply replace the server's private ip address with the external one. Or if you have forward and reverse pointers configured for your server's mx record mapped to your external ip address then you can use that instead. A name is generally easier to remember than a 12 digit number.
You can - if you wish - achieve further customisation. You can change its default logo to something more appropriate for your needs? The logo's stored in /usr/share/squirrelmail/images. Alternatively you could use the command line and issue:
sudo /private/etc/squirrelmail/config/conf.pl
Select the first option > Organisation Preferences. Adjust to suit. You can achieve further customisation - except for colour schemes - by editing the source files. You can display HTML and images by logging into Squirrel Mail and selecting Options > Display Preferences.
Hope this helps, Tony