To make your @icloud.com address the default address for sending email from your iCloud account, follow the steps below.
In iOS 6:
• Go to Settings > iCloud > Account (your Apple ID) > Advanced: Mail.
• Under “iCloud Account Information”, tap your email address.
• From the list of addresses available, tap the @icloud.com email address you would like to use as the default address for sending mail from iCloud Mail.
In OS X Mountain Lion or Lion v10.7.2 (or later):
• Go to Mail Preferences > Composing.
• Click the “Send new messages from” drop-down and select the @icloud.com email address you’d like to use.
• In Outlook 2007: Choose Tools > Account Settings > Email tab, select your @icloud.com email address from the list, and click “Set as default.”
• In Outlook 2010: Choose File > Info > Account Settings > Email Tab, select your @icloud.com email address from the list, and click “Set as default.”
• Go to the Mail app.
• Click the Action (gear icon) menu and choose Preferences.
• Under “Composing,” click the “Set a default address” drop-down and select the @icloud.com address you’d like to use.