Create a Conditional Access Policies to Block Legacy Authentication

Sign in to the Azure portal as a global administrator, security administrator, or Conditional Access administrator.
Browse to Azure Active Directory > Security > Conditional Access.
Select New policy.
Give your policy a name. We recommend that organizations create a meaningful standard for the names of their policies.
Under Assignments, select Users and groups
1. Under Include, select All users.
2. Under Exclude, select Users and groups and choose any accounts that must maintain the ability to use legacy authentication. Exclude at least one account to prevent yourself from being locked out. If you do not exclude any account, you will not be able to create this policy.
3. Select Done.
Under Cloud apps or actions, select All cloud apps.
1. Select Done.
Under Conditions > Client apps, set Configure to Yes.
1. Check only the boxes Exchange ActiveSync clients and Other clients.
2. Select Done.
Under Access controls > Grant, select Block access.
1. Select Select.
Confirm your settings and set Enable policy to Report-only.
Select Create to create to enable your policy.

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