Hi,
I had the same problem before and figured out why.
There is a program allowing sqwebmail to check the username & password which is authdaemon
Basicly, it runs when you start your server, I believe it's in rc.local or somewhere. But it dies after a while (the old one I used), please check it and run it, or write a script that will control if it's running. Without that program sqwebmail will not authenticate.
Since, I don't use redhat, I don't know the place of the program, simply runthe find command.
Best Wishes,
Kaan