Hotmail in Mail.app
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Whereas Gmail is fairly open and easy to implement into Mail.app, Hotmail is slightly more complicated if you don't have a Hotmail Plus account. Hotmail Plus offers access to your mail through Mail.app, and there is a workaround for standard Hotmail accounts, using FreePOPs.
If you have an MSN Hotmail Plus account, you can easily access your mail through Mac OS X Mail by installing the HTTP Mail Plugin.
Once installed, open mail and go to Preferences -> Accounts and create a new account with the following details:
Account Type: httpmail
Description: Hotmail (or something similar)
Email Address: email@example.com (your hotmail address)
Full Name: Your Name
Incoming Mail Server: hotmail.com
User Name: Your hotmail user name Password: Your Hotmail Password
Outgoing Mail server: None
Close the window and Save the Account.