iCloud is only backing up one email account. I have several email accounts and they are not backing up to iCloud even though I can read them on my phone.

How can I get iCloud to back up email from all of my email accounts? iCloud is only backing up email from one account. How do I get it to back up email from all of my email accounts? When I check my mail on my phone all of my emails appear from the different accounts but when I check iCloud only email appears from one account.