Watch the mail log file see if you get any errors when you attempt a login to squirrelmal. "tail -f /var/log/maillog" in your console should do it.
You could also try a "tail -f /var/log/httpd/error.log" or "tail -f /var/www/<domain>/log/error.log and watch to see if you get any errors from apache when you login in.
Is there any particular reason why you chose Fedora 15? According to Redhat site, official support for that ended 26JUN12. If its a Redhat derivative you're most comfortable with, perhaps look at CentOS 6.4 instead? Of course that won't automatically get your mail working but just something to consider.