The SQL UserName is the account used to access the @Mail database via MySQL. Verify the user has permission to access and alter the database via MySQL. The user 'root' is always allowed to create and alter databases so if you don't have a specific user for @Mail use user 'root'.
The SQL Password is the password corresponding with your SQL UserName. Make sure you have at least a password for user 'root ' or your specific @Mail user as not having a password will allow anyone to access this data. Telnet in and use 'mysqlaccess' to assign users and passwords to databases.
Use SQL Database Name is the name of the database @Mail uses. If this is your first installation of @Mail you should choose a name here and use the function Create Database Structure to create a clean @Mail database structure on your MySQL server. If you already have an @Mail database you can just fill in the name here.