8. Setting Up Apple Email Account

This is relevant if you have 2 factor authentication enabled on your Apple account.

https://support.apple.com/en-us/HT204397

Using app-specific passwords

App-specific passwords allow you to sign into your account securely when you use third-party apps with your Apple ID.

App-specific passwords are passwords for your Apple ID that let you sign into your account and securely access the information you store in iCloud from a third-party app. For example, use app-specific passwords with mail, contacts, and calendar services not provided by Apple. App-specific passwords maintain a high level of security and help ensure your Apple ID password won’t be collected or stored by any third-party apps you use.

To generate and use app-specific passwords, your Apple ID must be protected with two-factor authentication.

How to generate an app-specific password

Sign into your Apple ID account page.
In the Security section, click Generate Password below App-Specific Passwords.
Follow the steps on your screen.

After you generate your app-specific password, enter or paste it into the password field of the app as you would normally.

You are now ready to setup OneMail

In OneMail

Tap the icon in the upper left
Select "E-mail Settings
Select “Add IMAP Account”
Enter your email address and for the password use the one you just generated.

You are now ready to use OneMail to check your account.

Just return the OneMail inbox and tap the Connect button.