iCloud Backup
does backup your iPhone mail settings. By settings, I mean the login password, server path, and the like - the things you'll see at Settings > Mail, Contacts, Calendars.
This explains the contents of an iCloud backup.
https://support.apple.com/kb/PH12519 Just to be clear, a "backup" does not include data that's stored in iCloud in other ways - iCloud Contacts, iCloud Photo Library, Documents in the Cloud/iCloud Drive... these aren't part of the backup, because they're already stored elsewhere in iCloud (same idea for Gmail, Yahoo, Hotmail, etc. - that's already stored at Google, Yahoo, Microsoft, etc.). Also, a lot of stuff falls into the category of "app data" - just because it's not explicitly mentioned in the article does not necessarily mean it's not being backed up.
If you're looking for iCloud Backup to store the entire contents of your iPhone... not going to happen. It's just plain inefficient to duplicate info that's already stored somewhere else in the cloud. iCloud Backup stores the info necessary to
reconstruct the info on your iPhone.