If after installing SquirrelMail on IIS with default hMailServer settings (and testing the configuration seems okay) attempts to login may be refused.
This appears to occur if PHP scripts are handled by the CGI executable rather than the ISAPI DLL.
To change the PHP executable from PHP-CGI.exe to PHP5ISAPI.DLL do the following:
In IIS Manager select your Servers Properties, then edit the WWW Server Master Properties.
On the Home directory tab click Configuration.
Scroll to the entry for .PHP and edit it. Select the ISAPI dll as the Executable file, ie C:\PHP\php5isapi.dll.
If using IIS, make sure you set a session directory inside your PHP.INI. If you do not, your mail server will work for a while, and then deny valid logins after some time.