Integrated Security Success on Master, Fails on Other Database

Question

I'm a bit perplexed as to why my connection string is selectively succeeding on the master database in SQL Server 2005. Here are the details of my app. Any ideas where to look? I'm clearly missing something. Also keep in mind that the IIS Server and SQL Server are on different machines within an Intranet environment where I am attempting to implement Windows Authentication. Thanks. :)

Connection strings:

Server=SQLServer;Database=master;Integrated Security=true --succeeds Server=SQLServer;Database=other;Integrated Security=true --fails

Fail Error Message:

Cannot open database "other" requested by the login. The login failed. Login failed for user 'DOMAIN\IIS_SERVERNAME$'.

My IIS Server App Pool and Authentication settings:

• AppPool: .NET 4.0; Integrated Pipeline; Identity=Network Service
• Authentication: Impersonation=Disabled; Windows Auth=Enabled; Kernel-mode=Enabled; Providers=Negotiate,NTLM

Update: Clearly there is a Login of some sort for the machine connecting to my SQL Server with the Integrated Security, otherwise it wouldn't succeed in connecting to the master database right? I would guess that to mean there's an issue at the database level, but I cannot figure out which user account would give it access to master because there are only a handful of users for it.