In the Perfect Server set up guide it will of told you to call your server something like server1.domainname.tld . So the address to webmail would be http://server1.domainname.tld/webmail
Make sure DNS records are set up correctly, and propagated ( can take up 24-48 hours every time you make a change. You should have DNS records set up for server1.domainname.tld , you can check the current dns by doing a dnsdig at http://ftp.menandmice.com/knowledgehub/tools/dig
( leave "Name Server" blank )
If squirrelmail comes up when you use the IP address, then its installed and working ok, which means its most likely just a domain/DNS issue.