AD User Authentication

I am attempting to create an ASP.NET (.NET 3.5) website to connect to our Exchange 2010 server through Exchange Web Services, I am able to connect to EWS when I define the username, password and domain to authenticate with but I would like, if possible, to not include login details in my code.

In IIS I have enabled Integrated Windows Authentication for the site, in web.config of the site I have <authentication mode="Windows"/>.

The following code is what I have been woking with:

svc.UseDefaultCredentials = True
svc.Credentials = New WebCredentials()
svc.Url = New Uri(svcURL)

With the above code I am receiving the message:

When making a request as an account that does not have a mailbox, you must specify the mailbox primary SMTP address for any distinguished folder Ids.

When I attempt to use svc.Credentials = CredentialCache.DefaultNetworkCredentials (in place of svc.Credentials = New WebCredentials()) I receive the error message:

Unable to cast object of type 'System.Net.SystemNetworkCredential' to type 'Microsoft.Exchange.WebServices.Data.ExchangeCredentials'.

As mentioned, the only thing that has worked is to define the user credentials to authenticate to by hardcoding user login details, which I would rather not do: svc.Credentials = New WebCredentials("username","password","domain")

Has anyone been able to authenticate to EWS using the credentials of the current logged in user in an ASP.NET website?

By default it is not possible to delegate a user's credentials from one server (the server on which you are hosting your ASP.NET site) to another (your Exchange server). This is known as a "server hop" and Windows will prevent it by default as a security measure.

You have a couple of options to work around this:

Using Kerberos: When Kerberos is enabled it makes it possible to delegate user credentials between servers when using Windows authentication. I do not know the exact details on how to set up Kerberos as I am only a humble developer but maybe your system administrator can assist you. AFAIK, you need to set up your ASP.NET server to allow user delegation.
Setting the user identity of your IIS application pool: If Kerberos is not an option, you may change the identity of the application pool that your ASP.NET site is running under. First define a new application pool in IIS manager. Then go to the Advanced Settings dialog for that application pool and set the identity to a domain user that is allowed to access your Exchange server. More info on the application pool identity here: http://technet.microsoft.com/en-us/library/cc771170(v=WS.10).aspx.
Setting the <identity> element: If you for some reason cannot change the application pool, you may try impersonation of your ASP.NET web site using the <identity> element in your web.config file. ASP.NET gives you the option of storing the credentials in the registry so that you do not have to put them directly in your web.config file. More info here: http://msdn.microsoft.com/en-us/library/72wdk8cc(v=vs.90).aspx
Using the <appSettings> ellement and encryption: The last option I can think of is to simply put the credentials in your web.config file as normal <appSettings> and then encrypt the entire <appSettings> section. You would then simply read the appSettings from your code using the AppSettingsReader class. .NET allows you to encrypt sections of the web.config file out of the box and you can read the settings without event noticing that the section is encrypted. .NET takes care of decrypting for you. More info here: http://msdn.microsoft.com/en-us/library/zhhddkxy.aspx