icloud mail storage

As per this article https://support.apple.com/kb/ph2620?locale=en_US

If you don’t see onscreen instructions after turning on iCloud Mail on your iOS device or Mac, you already have a primary iCloud Mail email address. To create an email alias, see Use email aliases in Mail on iCloud.com.

After you set up your @icloud.com email address, you can use it to sign in to iCloud. You can still sign in using the Apple ID you used when you set up iCloud.

See the article https://support.apple.com/en-us/HT201320

Once iPhone is synced with iCloud the mails will get stored in iCloud server .