An administrator has a user who connects to Exchange 2007 using an IMAP account so that he can keep messages on multiple devices and he can work offline from multiple locations/devices as needed. The user is having problems subscribing to more folders and receives an error: "Failed to subscribe the folder. The server responded: ‘Server Unavailable. 15.'." This error occurs with both Outlook 2007 and 2010.
I'll address the error first. According to a contact in support, this is a limitation in Exchange 2007, not Outlook. The property Exchange 2007 uses to store subscribed folders is capable of storing about 32k of data, enough for about 500 folders. This issue was resolved in Exchange 2010. The user will not be able to add more folders using IMAP, unless you upgrade to Exchange 2010.
However, the user could solve the problem easily by using the native Exchange connectivity in Outlook. Instead of using an IMAP account, use cached Exchange mode. Cached Exchange was designed for scenarios just like this. The user will have access to everything in his mailbox, including calendar, contacts, and tasks both online or offline. Exchange cached mode is like IMAP in that it is a server-side mailbox and mail is synced with Outlook and a copy stored locally, not downloaded and deleted from the server as it is with a POP3 account. Plus, his "experience" will be much better with a cached Exchange account; while IMAP support in Outlook 2007 is much improved, it's far from perfect.
Users who are stuck with an IMAP account because they need two (or more) Exchange accounts should consider upgrading to Outlook 2010 as it supports multiple Exchange accounts.