Before you can use iCloud Mail on your iPhone, iPad, iPod touch, or Mac, you must first set up a primary iCloud email address on your device. See Send and receive iCloud Mail on all of your devices and keep Mail settings up to current for an overview of what you can do with Mail with iCloud.
If you already have a @icloud.com (or a @me.com or a @mac.com) email address, see Set up iCloud for Mail on all your devices for instructions on how to configure it on your other devices.
Create a primary iCloud Mail address on your iPhone, iPad or iPod touch
Using your iPhone, iPad, or iPod touch, set up a primary iCloud Mail account for yourself.
iCloud may be found in the Settings app on your iPhone, iPad, or iPod touch under [your name].
Once iCloud Mail (or Mail) has been enabled, proceed as directed by the onscreen prompts to create an iCloud Mail address.
In the event that you do not see any onscreen instructions after turning on iCloud Mail on your iPhone, iPad, or iPod touch, it is likely that you already have an active primary iCloud email account.
Create a primary iCloud Mail address on your Mac
Create a primary iCloud Mail address on your Mac and you’re good to go.
Using your Mac, go to the Apple menu > System Preferences, then select Apple ID, and finally choose iCloud.
Choose Mail from the drop-down menu, and then follow the onscreen directions to create an iCloud Mail account.
It is possible that you already have an iCloud Mail primary email address if you do not see any onscreen instructions after turning on the service on your Mac.