Some days ago my daughter started not to be able anymore to read her emails from her Lumia 950 smartphone because of synchronization problems. Moreover, every now and then, she also received an alert in the action center saying that “Le impostazioni dell’account iCloud non sono aggiornate” (ENG: “The settings of the iCloud account are not updated“).
I tried even to delete that email account from the Outlook Mail app and recreate it … nothing! The email account creation succeeded (so username and password were the right ones) but then the synchronization continued to fail.

The creation of the iCloud email account succeeded

I tried both on her Windows 10 Mobile smartphone and on my Windows 10 PC using the same universal Mail app: as expected, the problem was the same in both environments.

I then logged into the Apple site related to Apple ID management and suddenly, after entering username and password, I was asked for a 6 digits code that should be provided somewhere. No SMS arrived on my daughter smartphone (possibly she did not register any phone number in her Apple account) but, opening her iPad, there was there an alert related to a logon attempt from another device (my PC) and a request to possibly allow that access: once agreed, the code to be inserted to log into the Apple site was provided. Very probably if you have a registered iPhone, that code is provided through it and not necessarily on the iPad (that you could not have always with you 😉
So once logged into that site, you it must be checked the section related to the TWO-FACTOR AUTHENTICATION: if is set to active, it must be deactivated it in order to solve the iCloud email synchronization problem. When disabled there will not be neither the need anymore to get the 6 digit code when logging into the Apple site related to Apple ID management …

TWO-FACTOR AUTHENTICATION section in the Apple account management site

When requested to disable that Two-factor authentication, a notification email is sent to all the email accounts set in the Apple account management  site.

Notification email

Very probably some time ago a popup window appeared on my daughter iPad/iPhone asking to enable that more secure way to log in with the Apple account: however, agreeing on that, there is always the need to get the one time code and use it whenever you want to be authenticated … and also the authentication of some clients (e.g email ones like Outlook) could fail in their synchronization process.
In fact, for example, some Microsoft applications do not support that two-factors authentication (e.g. Microsoft Outlook, Windows live Mail, Xbox 360, Zune, apps that use EAS or POP3, other email clients). If you go into the Microsoft site page related to your account management, you will see in the Security& privacy section (More security settings) there is section related to the activation of a two-step verification (that is the same think called two-factor authentication in the Apple world …): you can find more details in this page. However, if you do not really need to have a very sicure account and it is sufficient for you to change every few months your password, I think that the two-factor authentication/two-step verification is only a unnecessary complication …

Two-steps verification in the Microsoft account management site

That’s all folks!